Hey guys. I'm quite new to ebikes in general. I was wondering if someone could help me with a little battery problem.

[KDavidson]

I'm planning on making an EMTB and for the battery solution I have decided to try and use 4 12V batteries in series. This one in particular. However it says that it has an amperage of 2A, which seems a bit low, but I don't know if that's a problem or a limitation...

Thanks guys.

-EDIT-

Clearly my initial plan was a terrible idea
Thanks for helping me not blow myself up. I have since looked at this, which is a 12000mAh 6s 12C battery. Would using 2 of these in series to get 44.4V work to power a system that is recommended for 48V. That is assuming that 12C is enough for an ebike.

Thanks again for any help.

 

[Wiltshire Warrior]

I would say that is a false economy, as after you have connected them up you will need to work out the charging and you won't have a BMS, so my advice would be to go for a rack mounted battery.

I am using a 400Wh rack system and its great this is the battery which has an integrated reflector and led lights:

It came with an alloy rack and charger for £250

 

[Mattwilko92]

I'm planning on making an EMTB and for the battery solution I have decided to try and use 4 12V batteries in series. This one in particular. However it says that it has an amperage of 2A, which seems a bit low, but I don't know if that's a problem or a limitation...

Thanks guys.

Depends what size motor you are using. I am no ebike builder, but an electrical engineer.

The current I in amps (A) is equal to the power P in watts (W), divided by the voltage V in volts (V):
I(A) = P(W) / V(V)

For example a 250W motor operating at 48v (4 x 12v batteries in series) has an operating current of 5.2 A.

 

[KDavidson]

I would say that is a false economy, as after you have connected them up you will need to work out the charging and you won't have a BMS, so my advice would be to go for a rack mounted battery.

I am using a 400Wh rack system and its great this is the battery which has an integrated reflector and led lights:

It came with an alloy rack and charger for £250

Thank You, but I was hoping to get more for the money

 

[KDavidson]

Depends what size motor you are using. I am no ebike builder, but an electrical engineer.

The current I in amps (A) is equal to the power P in watts (W), divided by the voltage V in volts (V):
I(A) = P(W) / V(V)

For example a 250W motor operating at 48v (4 x 12v batteries in series) has an operating current of 5.2 A.

It's a 1000W motor, however it will be running at 500W because of a speed limiter. So thats like 10-20Amps required. I'm not sure if the amps stack when the batteries are connected in series however, because if it does then I would be getting around 8Amps, which still isn't enough. Is there any possible way to get the right amperage?

 

[MattyB]

I don’t see any C rating (explanation here) in the specs of that pack, but to get enough current out of such a setup you will need at least 8 cells, not 4, wired as a 4S2P (2 sets of 4 series cells wired in parallel) pack. You may need 12 wired as a 4S3P. Either way you will need a decent balance charger and power supply (£50-100 minimum) to ensure you can charge it safely and keep the cells in balance, and as pointed out about you will have no protection against an over-discharge which will kill the pack permanently.

Short answer - You need knowledge to do this safely and decent soldering skills to create the pack. By the time you have done this and paid out for the kit you need you will be well over the cost of buying an aftermarket conversion kit. That is a better route IMO if you don’t want to buy a purpose built ebike.

 

[KDavidson]

I don’t see any C rating in the specs of that pack, but to get enough current out of such a setup you will need at least 8 cells, not 4, wired as a 4S2P (2 sets of 4 series cells wired in parallel) pack. You may need 12 wired as a 4S3P. Either way you will need a decent balance charger and power supply (£50-100 minimum) to ensure you can charge it safely and keep the cells in balance, and as pointed out about you will have no protection against an over-discharge which will kill the pack permanently.

Short answer - You need knowledge to do this safely and decent soldering skills to create the pack. By the time you have done this and paid out for the kit you need you will be well over the cost of buying an aftermarket conversion kit. That is a better route IMO if you don’t want to buy a purpose built ebike.

I'm not sure what you mean. Correct me if I'm wrong, it is 4 12V batteries, not cells. And I can charge the batteries separately because they each have a charge port. My main concern is getting a decent amount of amps out without the overheating or worse.

 

[MattyB]

I'm not sure what you mean. Correct me if I'm wrong, it is 4 12V batteries, not cells. And I can charge the batteries separately because they each have a charge port. My main concern is getting a decent amount of amps out without the overheating or worse.

Yes, you are correct - each 12V battery pack is made up of 3 cells in series (a 3S1P configuration). That means if you put 4 in series you are actually making a 12S1P pack. Such a pack would have a theoretical capacity of 48X20 = 960Wh. So if we can get sufficient currrent out of it that would appear to be enough capacity - a bit less than 2hrs running at 500W.

We don’t know the C rating of these packs, but given their price I think it is safe to presume they won’t be very high. Let’s go with a max constant discharge of 1C (the best cells are rated at 3-4x that according to specs, but these are eBay cheapies so let’s not push them too hard), which gives a max current of 20A (it’s a 20Ah battery). To ensure decent cycle life though you should probably halve that to 10A. Based on that you should be able to deliver roughly 480W (48V X 10A) constant safely to the motor from such a setup, getting maybe 75% of that at the wheel. You will need to decide if that is sufficient.

I guess I was being conservative when I thought you would have to parallel them up, but I still think the likely cycle life of these packs will be short - they are almost certainly designed for long, slow discharges at low currents, and this usage pattern isn’t that. You might even find the voltage completely collapses under peak loads if the cell quality is poor and the internal resistance very high. Maybe buy one and test it’s behaviour against a variable load to see how it performs?

 

[MattyB]

Ps - Another concern... you have no idea if the internal wiring of these packs is rated for the currents we are talking about. I can certainly tell you from RC that no-one uses plugs like that for high current applications, so they will need to be removed and replaced with something like an XT60 or Deans connector. Remember, lithium fires are not fun and you will be charging this in your house and discharging it between your legs so you need to check it carefully. Dissembling a test pack for inspection would seem sensible if you are serious about going this route.

 

[KDavidson]

Ps - Another concern... you have no idea if the internal wiring of these packs is rated for the currents we are talking about. I can certainly tell you from RC that no-one uses plugs like that for high current applications, so they will need to be removed and replaced with something like an XT60 or Deans connector. Remember, lithium fires are not fun and you will be charging this in your house and discharging it between your legs so you need to check it carefully. Dissembling a test pack for inspection would seem sensible if you are serious about going this route.

Thanks. I have edited my initial post because I have realised that my batteries are not a good idea.

 

[KDavidson]

Yes, you are correct - each 12V battery pack is made up of 3 cells in series (a 3S1P configuration). That means if you put 4 in series you are actually making a 12S1P pack. Such a pack would have a theoretical capacity of 48X20 = 960Wh. So if we can get sufficient currrent out of it that would appear to be enough capacity - a bit less than 2hrs running at 500W.

We don’t know the C rating of these packs, but given their price I think it is safe to presume they won’t be very high. Let’s go with a max constant discharge of 1C (the best cells are rated at 3-4x that according to specs, but these are eBay cheapies so let’s not push them too hard), which gives a max current of 20A (it’s a 20Ah battery). To ensure decent cycle life though you should probably halve that to 10A. Based on that you should be able to deliver roughly 480W (48V X 10A) constant safely to the motor from such a setup, getting maybe 75% of that at the wheel. You will need to decide if that is sufficient.

I guess I was being conservative when I thought you would have to parallel them up, but I still think the likely cycle life of these packs will be short - they are almost certainly designed for long, slow discharges at low currents, and this usage pattern isn’t that. You might even find the voltage completely collapses under peak loads if the cell quality is poor and the internal resistance very high. Maybe buy one and test it’s behaviour against a variable load to see how it performs?

Thank you so much. Could you check my edit for the new batteries I am looking at and advise

 

[MattyB]

-EDIT-

Clearly my initial plan was a terrible idea
Thanks for helping me not blow myself up. I have since looked at this, which is a 12000mAh 6s 12C battery. Would using 2 of these in series to get 44.4V work to power a system that is recommended for 48V. That is assuming that 12C is enough for an ebike.

Thanks again for any help.

It’s definitely a better bet. Some key points:

1) This is a lipo, a slightly different variant of the li-ion chemistry used in most ebikes. They can be charged and discharged quicker, but are less safe and generally have lower cycle life - pushed hard in RC 100-200 cycles is considered pretty good, though there are some variants that do much better than this. Nominal voltage is 3.7V/cell, but at full charge they are 4.2V/cell. That means at full charge your 12S pack would have 50.4V, so fine for your motor. You do need to make sure you do not store them at full charge though as they are even more vulnerable to being damaged by that than the packs in commercial ebikes.

2) The capacity of your packs in a 12S1P config is nominally 3.7 X 12 X 12 = 530Wh, which seems pretty sensible. The C ratings of the lipos are notoriously exaggerated (especially Hobbyking ones), so I always work on half the labelled value being a safe bet. Even so at 6C we are talking 6 X 12A = 72A max discharge; that means the peak power the battery could support with the appropriate load would be roughly 3.7 X 12 X 72 = >3kw. 1kw should not be a problem!

3) These are soft pouch cells, not hard case. As such you must add a hard case when using them on the bike, pad them out inside for their own protection in a crash, and make sure there is a bit of ventilation of the case to allow heat to escape in use (though at the powers you are using heat build up should be very minimal). Lipo fires are not fun, so how you protect the cells on the bike needs careful thought. You’ll also need to be able to open up the case to charge the cells in parallel off the bike (see below).

4) You will need at least a 6S capable lipo charger (minimum rating 300W if you need to be able to charge from flat in 4hrs; the Turnigy Reaktor range is good and affordable), a DC power supply (rated to powers at least as high as the charger - converted server PSUs are best for that) and balance board. I would also recommend a parallel charge board so that you can charge both packs in parallel at lower charge rates (safer and better for the batteries long term) and an insulated ammo box or similar for charging in for safety. Lipo fires are nasty and difficult to extinguish, so never charge unattended and make sure you have a suitable extinguisher to hand when charging. Google lipo charging setups and you should start to get a feel for what you need.

Hope that helps! Good luck, and remember, safety first at all times - you can store a lot of energy in these packs (all these demos are on batteries far smaller than yours)...

 

[MattyB]

PS - A single over discharge of these packs to below 3V/cell can kill them permanently . You will therefore need to be very conservative on usage initially until you understand how much battery you are using, but even better would be fitting some kind of battery monitoring system. A crude solution would be an alarm like this plugged into the balance port on each battery pack; a better one might be a display like this on the bars...

HobbyKing™ Battery Medic System (2S~6S)

 

[KDavidson]

It’s definitely a much better bet. Some key points:

1) This is a lipo, a slightly different variant of the li-ion chemistry used in most ebikes. They can be charged and discharged quicker, but are less safe and generally have lower cycle life - pushed hard in RC 100-200 cycles is considered pretty good, though there are some variants that do much better than this. Nominal voltage is 3.7V/cell, but at full charge they are 4.2V/cell. That means at full charge your 12S pack would have 50.4V, so fine for your motor. You do need to make sure you do not store them at full charge though as they are even more vulnerable to being damaged by that than the packs in commercial ebikes.

2) The capacity of your packs in a 12S1P config is nominally 3.7 X 12 X 12 = 530Wh, which seems pretty sensible. The C ratings of the lipos are notoriously exaggerated (especially Hobbyking ones), so I always work on half the labelled value being a safe bet. Even so at 6C we are talking 6 X 12A = 72A max discharge; that means the peak power the battery could support with the appropriate load would be roughly 3.7 X 12 X 72 = >3kw. 1kw should not be a problem!

3) These are soft pouch cells, not hard case. As such you must add a hard case when using them on the bike, pad them out inside for their own protection in a crash, and make sure there is a bit of ventilation of the case to allow heat to escape in use (though at the powers you are using heat build up should be very minimal). Lipo fires are not fun, so how you protect the cells on the bike needs careful thought. You’ll also need to be able to open up the case to charge the cells in parallel off the bike (see below).

4) You will need at least a 6S capable lipo charger (minimum rating 300W if you need to be able to charge from flat in 4hrs; the Turnigy Reaktor range is good and affordable), a DC power supply (rated to powers at least as high as the charger - converted server PSUs are best for that) and balance board. I would also recommend a parallel charge board so that you can charge both packs in parallel at lower charge rates (safer and better for the batteries long term) and an insulated ammo box or similar for charging in for safety. Lipo fires are nasty and difficult to extinguish, so never charge unattended and make sure you have a suitable extinguisher to hand when charging. Google lipo charging setups and you should start to get a feel for what you need.

Hope that helps! Good luck, and remember, safety first at all times - you can store a lot of energy in these packs (all these demos are on batteries far smaller than yours)...

That was a very informative read. Thank you very much. A few questions, however.
How long should 100-200 charge cycles last me, aproximately in years?
I have been looking at charging solutions and had landed on the IMAX B6, which is a very common and cheap charger. Would you recommend still going with the Reaktor range (which I am currently looking into), or should the IMAX suffice?
I have already researched and chosen a parallel charger so I think I should be alright there.

Thanks in advance?

 

[MattyB]

That was a very informative read. Thank you very much. A few questions, however.
How long should 100-200 charge cycles last me, aproximately in years?
I have been looking at charging solutions and had landed on the IMAX B6, which is a very common and cheap charger. Would you recommend still going with the Reaktor range (which I am currently looking into), or should the IMAX suffice?
I have already researched and chosen a parallel charger so I think I should be alright there.

Thanks in advance?

Range depends on lots of variables, but most on here are getting 20-30 miles out of a 500Wh battery like yours. Probably given the low discharge rates you’d get more that 200 cycles out of your pack, so let’s say 300... if you did a full discharge ride twice a week, we’d be looking at ~150 weeks or just under 3 years. Increase your ride frequency and that will obviously drop.

Re: The charger, no, the IMAX B6 is not even close to what you need - it is only 50W rated with a max current of 5A, so charging up your packs would take the best part of a day! Look for something with at least a 300-500W rating on a 24V PSU so you can charge both your packs in parallel at a decent rate. This guide is worth reading to explain why we parallel charging is useful and how to do it.

LiPo Battery Parallel Charging Tutorial - Oscar Liang

 

[KDavidson]

PS - A single over discharge of these packs to below 3V/cell can kill them permanently . You will therefore need to be very conservative on usage initially until you understand how much battery you are using, but even better would be fitting some kind of battery monitoring system. A crude solution would be an alarm like this plugged into the balance port on each battery pack; a better one might be a display like this on the bars...

HobbyKing™ Battery Medic System (2S~6S)

I was actually looking at this to watch battery usage, but it's good to know that I shouldn't go below 3V. Could you confirm that the Cellmaster is appropriate.

 

[KDavidson]

Range depends on lots of variables, but most on here are getting 20-30 miles out of a 500Wh battery like yours. Probably given the low discharge rates you’d get more that 200 cycles out of your pack, so let’s say 300... if you did a full discharge ride twice a week, we’d be looking at ~150 weeks or just under 3 years. Increase your ride frequency and that will obviously drop.

Re: The charger, no, the IMAX B6 is not even close to what you need - it is only 50W rated with a max current of 5A, so charging up your packs would take the best part of a day! Look for something with at least a 300-500W rating on a 24V PSU so you can charge both your packs in parallel at a decent rate. This guide is worth reading to explain why we parallel charging is useful and how to do it.

LiPo Battery Parallel Charging Tutorial - Oscar Liang

I was planning on a 16 mile commute daily on weekdays so, how would this affect the 3 year approximation you gave?

Also, would this be enough as a charger and what amperage should I be charging at. I was thinking 12A, but I would like a more educated opinion.

 

[MattyB]

I was actually looking at this to watch battery usage, but it's good to know that I shouldn't go below 3V. Could you confirm that the Cellmaster is appropriate.

It would work but if you want to monitor them in real time you’d need two, one to monitor each pack.

 

[KDavidson]

It would work but if you want to monitor them in real time you’d need two, one to monitor each pack.

Yes, of course, thanks.

 

[MattyB]

I was planning on a 16 mile commute daily on weekdays so, how would this affect the 3 year approximation you gave?

Also, would this be enough as a charger and what amperage should I be charging at. I was thinking 12A, but I would like a more educated opinion.

16 miles will probably be between 50-70% of a full discharge, so do the rest of the maths and you can work out how long it would last in terms of time.

Re: charging, yes that should be fine and you can just about achieve 12A based on a charge voltage of 25V (25 X 12 = 300W). Charging both packs in parallel at that rate would give you a recharge time of ~2hrs.

 

[KDavidson]

16 miles will probably be between 50-70% of a full discharge, so do the rest of the maths and you can work out how long it would last in terms of time.

Re: charging, yes that should be fine and you can just about achieve 12A based on a charge voltage of 25V (25 X 12 = 300W). Charging both packs in parallel at that rate would give you a recharge time of ~2hrs.

I calculated a minimum of two years, I don't know if my calculations were correct but, if anything, they were severely undercutting so I am alright with the lifespan. Thank you for the help with the charger and everything else

 

[KDavidson]

16 miles will probably be between 50-70% of a full discharge, so do the rest of the maths and you can work out how long it would last in terms of time.

Re: charging, yes that should be fine and you can just about achieve 12A based on a charge voltage of 25V (25 X 12 = 300W). Charging both packs in parallel at that rate would give you a recharge time of ~2hrs.

So that charger was a little too expensive and I was trying to cut down on costs. I came across the SkyRC B6 Nano. It can go up to 350W and 15A so it seems sufficient. Please advise if this is a good choice.

Also a slightly cheaper battery was this MultiStar. Please tell me if you see any problems with these changes thanks so much.

 

[MattyB]

Never seen one of those chargers, but the numbers are in the right region. Would not recommend the Multistar packs though - several of my friends who have used them in RC say they have high IR and therefore much lower C ratings than even the relatively low advertised values.

Ps - What PSU are you using? None of these DC chargers can be used off the mains. I suggest you read up on converting a server PSU; they are cheap, powerful and super reliable compared to consumer units.

How to convert Server Power Supplies - RC Groups

Convert HP server power supply for RC use - TJinTech

 

[Mabman]

Just get one of these type and be done with it:

Sell Well New Type Lithium Battery 48v 15.6ah Folding Ebike Battery - Buy Ebike Battery,Ebike Lithium Battery 48v 30ah,Ebike Conversion Kit Battery Product on Alibaba.com

48v, safe chemistry, easy to fit on the bike and will last for quite awhile.

 

[KDavidson]

Never seen one of those chargers, but the numbers are in the right region. Would not recommend the Multistar packs though - several of my friends who have used them in RC say they have high IR and therefore much lower C ratings than even the relatively low advertised values.

Ps - What PSU are you using? None of these DC chargers can be used off the mains. I suggest you read up on converting a server PSU; they are cheap, powerful and super reliable compared to consumer units.

How to convert Server Power Supplies - RC Groups

Convert HP server power supply for RC use - TJinTech

I have gone back to the original battery as per your recommendation.

I am entirely lost when it comes to this power supply stuff. I thought I could just splice in a 12V DC wall plug...
What is the cheapest way to do the power supply. As you can tell I am on a very tight budget.

At around 0:43 in this video about the B6 Nano, he says that it can be powered by a car cigarette lighter. This, as far as I am aware, tends to run on 12V. So this my thinking behind the wall plug.

 

[KDavidson]

Just get one of these type and be done with it:

Sell Well New Type Lithium Battery 48v 15.6ah Folding Ebike Battery - Buy Ebike Battery,Ebike Lithium Battery 48v 30ah,Ebike Conversion Kit Battery Product on Alibaba.com

48v, safe chemistry, easy to fit on the bike and will last for quite awhile.

That's what I was originally looking at. Of course not that exact one, but the LiPo's better fit my form factor and that seems far too cheap for 15Ah. Alibaba isn't usually a website that I trust..

 

[MattyB]

...I am entirely lost when it comes to this power supply stuff. I thought I could just splice in a 12V DC wall plug...
What is the cheapest way to do the power supply. As you can tell I am on a very tight budget.

At around 0:43 in this video about the B6 Nano, he says that it can be powered by a car cigarette lighter. This, as far as I am aware, tends to run on 12V. So this my thinking behind the wall plug.

A 12V DC wall plug that can supply 350W? Errrr...

Please take this in the spirit it’s meant... It is clear from your posts that you don’t have the necessary knowledge to do this safely at the present time. If you really want to home brew a setup you need to build up some basic electrical knowledge from core principles, stuff like V=IR, P=VI etc. Put simply it’s time to hit the books/Google! You are also going to need to be able to solder connectors, which on large wires requires some practice. Finally you need to build your knowledge of lipos - soft pouch lithium batteries must be charged and handled carefully, as charging errors or over-discharge can be fatal to the pack. At the moment I am concerned you are going to attempt this and have a nasty accident either on the bike or (more likely) during charging.

Personally I think you would be far better buying a ready made aftermarket lithium ion pack with inbuilt BMS as suggested above by @Mabman. Even if bought from a UK supplier by the time you have bought a 50-100W soldering iron, charger, PSU etc for your lipo setup it is unlikely to cost any more, and will be a lot safer. A friend of mine had his garage burnt down by a lipo charging fire, and he was very experienced - please don’t try and do this on the cheap without the necessary basic knowledge.

 

[KDavidson]

A 12V DC wall plug that can supply 350W? Errrr...

Please take this in the spirit it’s meant... It is clear from your posts that you don’t have the necessary knowledge to do this safely at the present time. If you really want to home brew a setup you need to build up some basic electrical knowledge from core principles, stuff like V=IR, P=VI etc. Put simply it’s time to hit the books/Google! You are also going to need to be able to solder connectors, which on large wires requires some practice. Finally you need to build your knowledge of lipos - soft pouch lithium batteries must be charged and handled carefully, as charging errors or over-discharge can be fatal to the pack. At the moment I am concerned you are going to attempt this and have a nasty accident either on the bike or (more likely) during charging.

Personally I think you would be far better buying a ready made aftermarket lithium ion pack with inbuilt BMS as suggested above by @Mabman. Even if bought from a UK supplier by the time you have bought a 50-100W soldering iron, charger, PSU etc for your lipo setup it is unlikely to cost any more, and will be a lot safer. A friend of mine had his garage burnt down by a lipo charging fire, and he was very experienced - please don’t try and do this on the cheap without the necessary basic knowledge.

Right well... OK. I think you are correct. It has been too expensive so far and I think it would be better to just go the prebuilt route. Thanks for your help, everyone.

 

[MattyB]

Np - I think that is the right decision. Best of luck with your conversion, and don’t forget to post some pics once it is done