Battery care/management

[Matt]

Just wondering what people’s thoughts are on the following...short ride today battery still near full, longer ride planned for tomorrow. Do you guys stick the battery on charge overnight just to top it up or leave till after ride tomorrow?

 

[Cris]

Lithium batteries don’t have the issue of “memory” when topping up a charge like you used to get on ni-cads, so i’d say fill it up. If it was being left for a week definitely don’t change it as lithium batteries are happiest being stored at 30-60%. And keep them in a heated area at this time of year, they don’t really like going below 10c, and never try to charge them at temperatures below 0c.

 

[Matt]

Cheers Cris, good advice I’m sure.

 

[Mad Mark]

Re charge however little I've used it ?

 

[MattyB]

This thread should help - the key thing is to make sure you never store the battery at an elevated state of charge (i.e. >80%) for anything longer than a day or so. That is what really kills lithium chemistry batteries.

Charging methods - battery health - EMTB Forums

 

[Mad Mark]

So you should leave it as it is till the day before you go out then / then don't forget to charge it...thanks for that matty ?

 

[Mattwilko92]

Crack on, the lithium batteries have a BMS (Battery Management System) to prevent overcharging.

 

[Mad Mark]

Yes think I'll stick to my way of doing it / that way when I go to them, they are charged ?

 

[MattyB]

Crack on, the lithium batteries have a BMS (Battery Management System) to prevent overcharging.

The BMS will prevent an over charge of course, but that is not the point. If you leave a lithium chemistry battery for extended periods at full or near full it promotes dendrite growth at the anode. Over time this will shorten cycle life, reduce capacity, increase internal resistance (and therefore the packs ability to deliver high currents) and in extreme circumstances can even cause an internal short, though this is less likely in ebike batteries than the lipos commonly used in RC vehicles.

How to Prolong Lithium-based Batteries - Battery University

 

[MattyB]

Yes think I'll stick to my way of doing it / that way when I go to them, they are charged ?

Fair enough, but don’t expect your pack to get anywhere near it’s quoted cycle life of you do that. Ask Nissan, they know this all to well from the 1st gen Leaf...

BU-808b: What Causes Li-ion to Die? – Battery University

 

[Mad Mark]

Fair enough, but don’t expect your pack to get anywhere near it’s quoted cycle life of you do that. Ask Nissan, they know this all to well from the 1st gen Leaf...

BU-808b: What Causes Li-ion to Die? – Battery University

I understand what you are saying matty, but I generally go out on my bike twice a week, so as I understand what you have said that shouldn't be a problem
What you seem to be saying is not to leave them fully charged for a long period of time......which is now understood
I will not charge them if I know I won't be going out on the bike for longer than a week
Thank you for your knowledge ?....we are always learning

 

[MattyB]

I understand what you are saying matty, but I generally go out on my bike twice a week, so as I understand what you have said that shouldn't be a problem
What you seem to be saying is not to leave them fully charged for a long period of time......which is now understood.

Sorry, but you haven’t quite understood my point. It is the total time spent at elevated charge that matters - the longer that is, the more the anode degradation occurs. Let’s compare your example with an alternative charging regime for the same pattern of usage:

Your example: Let’s assume you do a 3hr ride twice a week to fully discharged, then recharge immediately when you get home (which for most 500Wh bikes seems to take about 4hrs). That means your time at full charge per week is approximately (24x7)-(2x7)=154 hours per week, or just under 92% of the time. If you do more rides this % will decrease slightly, but less rides (such as can often occur in the depths of winter) will increase it. Either way the battery is spending the huge majority of it’s life at elevated SOC where internal degradation occurs at the fastest rate.

Alternative example: Now let’s look at another option for the same two 3hr rides; charging the night before. If we put the battery on at 5pm it should be fully charged by 9pm. Let’s go worst case and assume the ride on average will start at 2pm the next day. That means the battery will spend 2x17=34hrs per week at full charge, or roughly 20% of the time. Again if you miss a ride this % will obviously increase, but it won’t get anywhere near the 92% number from the previous example unless you charge then don’t ride for a week. Over an extended period this charging regimen would put substantially less stress on the battery, increasing cycle life and reducing the rate at which capacity is lost compared to charging back to full straight away. Caveat - It’s not a good idea to leave your pack at fully discharged for any significant length of time either. The easiest way to address that is to stick your battery on charge for say 30-60 mins on your return if you do fully drain it on a ride.

Alternative 2 - If you don’t generally drain your battery fully every ride there is an option which should marry the convenience of the first example with the improved battery life of the second. Buy a timer plug and set it to give you a charge between 70-75% of a full charge, then charge back up to that straight after your last ride. This means you will be ready for your “standard” ride all the time; if you want to do a longer ride you’ll need to top up for an hour or so beforehand. This is a great option for most people, but probably not if you have a bike with a smaller capacity pack like the Focus’ or upcoming EZesty where most rides will fully drain the pack.

 

[MattyB]

One final point... There is no definitive “best” way to handle charging of lithium packs. It all depends on your usage pattern and the balance the owner wants to strike between convenience and cycle life/battery health. If you ride every day and fully drain the pack every time then don’t worry too much about it, just charge straight up to full. Weekend warriors though should definitely think about their charging strategy if they don’t want to be replacing their pack prematurely at significant expense.

 

[Mad Mark]

I'm with you now ?

 

[MattyB]

I'm with you now ?

Np, hope it helps you.