Please tell me how to balance new ebike batteries

[Dave Sloan]

I have two batteries for my new bike — 48V, 17A Samsung. They don’t run in parallel, only one at a time. Both have the same problem. When they get under 50% on the display, they suddenly start to drop ten or fifteen percentage points at a time, then from 15% or so right to 0%, then, while still putting out low power, the display shows the battery jump back up to 4% or 8%. After that the battery might still put out low power for a long time. It might put out low power for an hour while showing 0%, or for an hour while showing 4% the whole time. The bottom line is that I can’t count on these batteries once they get under 50%.
I’ve been trying to fully cycle the batteries and let them stay on the charger for awhile when full. I’ve done that three or four times with each battery and had no improvement. I can’t find battery balancing instructions on the web. Can anyone help? Thanks so much!

 

[rich c]

Sounds like voltage sag to me. Does the display come back up when you stop? Some voltage measurements would help troubleshoot the packs.

 

[smorgasbord]

It might put out low power for an hour while showing 0%, or for an hour while showing 4% the whole time.

Don't do that! You are hurting your battery by forcing power from it at a low State of Charge. Your bike's controller should be smart enough to shut off to protect the battery, but it doesn't appear to be doing that.

Remember:

• Battery bar gauges are very crude measuring devices, based on voltage.
• Voltage itself is a crude estimate of battery State of Charge (SoC)
• Voltage under load (you did "while still putting out power") will be less, and not relevant to remaining battery capacity.

The end result is that battery bars have no hope of showing any kind of linear charge depletion. Here's a chart of voltage to SoC for 48 volt batteries:

And even this chart is crude, showing 5% SoC for every 0.8 volt difference. Reality isn't that linear. If you've been riding, you need to wait at a standstill for a minute before measuring voltage.

The reality is that you'll get more miles out of your first of five bars than the last of five bars. It's not uncommon to get, say, 10 miles on the first bar, 8 on the second, then 5 on the third, and just a few on each of the remaining two.

 

[harryS]

To balance the battery, you leave them on the charger for hours and hours, but it only works if your batteries have balance circuits.

I would expect that if a battery has name brand cells, that it also is capable of balance, but that's not always true. I see you bought a BPM with the Bafang Ultra motor with a spare pack, and you're got terrible mileage from one (7 miles) and mediocre from the other (20 miles). Ask the BPM guys if their batteries have balance circuits.

You probably need a voltmeter to really analyze this problem. You can also put a wattmeter in between the charger/battery. It will tell you how many AH was needed to recharge. It will also show if any balance current is flowing after full charge is reached. In my experience, they read voltage but are not that accurate. Adapter plugs would keep you from having to cut the charger cable and void warranty.

These are for reference only. No recommendation. Assumed your charger uses a 5.5mm barrel plug.

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[Alex M]

I'm a bit confused by your batteries do not run in parallel, so do they run in series?

I think he meant they are not running both at the same time. No parallel or series wiring here.

Anyway, he is killing the batteries by running them to 0%. Shouldn't let it drop below 20-30%.

As to the balancing, cheap-er BMS (read - most batteries on the market) only balance the cells when charged to near 100%, but to prolong their life it is desirable only occasionally letting it go to 100% to do the balancing, and charge to 80-85% rest of the time.

 

[Dave Sloan]

Thanks for the replies. The batteries are doing well until they get down to 50% charge. Then they get very erratic. Juice cuts off and then back on, as described above. Display readings drop suddenly and precipitously and then come come back, again, as described above. I experienced nothing like this with my Bosch motor/battery on my previous bike

I'm a bit confused by your batteries do not run in parallel, so do they run in series?
If it's 48v and 17ah, I'm assuming that's 4 parallel and 13 series???

I was also guessing what you meant by was that your battery does not keep constant voltage, but they do drop in voltage as you go.
The 48V battery for Li-Ion is 54.6V at 100%, and will drop down to 39V by the time you get down to 0.

One battery is a spare. I only use one at at time.

. The readings on the display dropped steadily. I always knew how much juice was left in the battery. With this battery, I get off the bike with 4% left in the battery and when I look at the display an hour later, without charging, it says I have 18%. I’m lost.
What I need to know now is what to do. I’ll ask bpm is the batteries have balancing circuitry. But how should I charge them?
BIG QUESTION: What percentage should I discharge to before recharging?
I was under the apparently false understanding that I should run them through full cycles in order to balance.
Related question: How can I know when to recharge if the display level is not trustworthy below 50%.

 

[City Commuter]

Thanks for the replies. The batteries are doing well until they get down to 50% charge. Then they get very erratic. Juice cuts off and then back on, as described above. Display readings drop suddenly and precipitously and then come come back, again, as described above. I experienced nothing like this with my Bosch motor/battery on my previous bike


One battery is a spare. I only use one at at time.

. The readings on the display dropped steadily. I always knew how much juice was left in the battery. With this battery, I get off the bike with 4% left in the battery and when I look at the display an hour later, without charging, it says I have 18%. I’m lost.
What I need to know now is what to do. I’ll ask bpm is the batteries have balancing circuitry. But how should I charge them?
BIG QUESTION: What percentage should I discharge to before recharging?
I was under the apparently false understanding that I should run them through full cycles in order to balance.
Related question: How can I know when to recharge if the display level is not trustworthy below 50%.

Your battery should have the proper cut out circuitry to shut down your battery before it gets too low, make sure it does.
In general, use the 80-20% rule, try not to discharge below 20% or recharge over 80%. It does not have to be strictly adhered to, but use it as a general rule of thumb to make your battery last longer over it's lifetime.

Once or twice a month when I plan on a long ride, I leave my battery on the charger overnight so it hits 100% and the cells balance out. That next morning I'm riding and letting that stored energy come back down again. That way the battery does not sit for days or weeks at 100%.

Like others have said, make sure those Samsung's have proper regulating circuitry so they don't get damaged. They're great batteries, I have two myself now.

 

[smorgasbord]

With this battery, I get off the bike with 4% left in the battery and when I look at the display an hour later, without charging, it says I have 18%. I’m lost.

What does the display say about 3 minutes later? As noted above, voltage drops when the battery is being used.