Help! Batteries discharging quickly

Hey everyone,

My ebike was working fine a few weeks ago, id get 30kms/charge, id plug it in every night and use it every morning.

I'm experiencing a problem now when I accelerate the bike hits its low voltage cutoff (74v) within seconds, I can barely get up to 20km/h before it happens. My voltmeter then jumps back up to around 91v after (92.6v is the max after leaving it fully charged for a few hours)

I tried charging it again and looked at all the wiring and everything looks ok.

Any idea where I should look next? Anyone experience this before?

Thanks in advance!

Sounds like old and/or crappy cells dying. Most likely you'll need to replace all the cells in the pack. Probably cheaper and easier to buy a whole new battery pack.

You could load test individual cells by using a big resistor to provide a load and watch the voltage sag. Repeat test for each cell group. Possibly only one is bad. Another test is to run it at as much load as it can handle for a few minutes then feel for anything that is heating up.

If it's dropping 17v under load it's not just one cell, unless it's actually reversing voltage.

Not knowing what chemistry it is, dunno how many volts per cell that is, but it's a huge drop to go from essentiallly fully charged to dead (LVC) in only seconds, and probably means the pack itself is old and worn out, or if it's new it means the cells are probably junk that weren't rated high enough for the load they had to supply, and are now damaged, but either way they are no longer capable of outputting the current the controller is asking for anymore.

Maybe something as stupid as a loose power wire somewhere?

Otherwise, open the pack.. and get to measuring.

Thanks for the tips guys.

More info:

The batteries are all less than a year old 7 12v 20ah SLAs in series. The bike was just manufactured last year as well. All packs measure ~13.4v individually after charging.

I put the bike on a stand and put the throttle to max, it ran for 70kms before starting to die (all lights on, playing with the throttle and brakes occasionally. I fully charged it and just went for a ride, I can ride normally but only for 6kms before the LVC starts cutting in while I'm accelerating. Which is a lot better than earlier but no where near the 30kms I got a few weeks ago. All connectors, batteries and cables are secure, no bulging, funny smells or any physical damage I can see. Just hit 1850kms

Any other tips?

Most likely it's just dead lead. Under load it sags too much in voltage because it's getting old, pushed too hard for it's age. Works ok when not loaded, because it's not pulling as much current.


It's *possible* there is a poor connection in one of the series connections between the batteries, but if they all bolt together, it'd probably be a crimp on one of the lugs that's not done right. If they're the slide-on type of connector, could be one of them is spread open just enough to make a high resistance connection (or again, could be a bad crimp).

More likely to be dead lead, though; it just sounds like how those things have died for me. Most of these things don't last even a hundred cycles; 50 if you're not so lucky.

Ah, lead-acid. Different animals.

Sometimes they get sulphated. Charge for about a week and see if there is any improvement. They could also be dried out and need water, but they are sealed and not easy (or safe) to add water to. But I have done it before with good results.

Load testing the individual batteries is still a good idea.

I ran lead for my first year of e-biking. They lasted about a year. Pull them apart and test each of them individually under load. Your local auto shop has one of those machines if you don't. Or maybe purchase on online for $25.00 USA plus shipping.

Lead-acid cells just don't have the longevity in them like lithium dose.

Edit: If they are all good, then check your wiring.

It is not necessary to load test separately. You can do the whole string at once with by connecting to something like a heater. I use one or two of those oil filled Delonghi ones, or light bulbs. Then periodically take the voltage of each battery and record over time to see if any are seriously lagging the others. Also, if it is getting cold then of course your batteries can't put out much energy.

You can also check for bad connections by using your voltmeter to detect voltage drops when the battery is under load.

Lead is dead, when they sag so bad
Time to forget them, heavy and dirty
Buy a real battery, and be happy

I bet they sagged a lot when they were new..
A continuous draw over 1C is usually too much.

Lead acid is getting more expensive by the year and has an extremely short life.. dunno why anyone is installing or using it in 2018.

I'm going to try another ride tonight. I know lead is old tech but its what it came with. Can anyone recommend a lithium or more reliable pack in the 84-96v 20+ah capacity?

thanks for the advice everyone

You might need to buy two or three battery packs from a reputable dealer and run them in series. It might take some custom building or mod on the bms depending on the wiring. RC lipo is also an option. Just make sure you are careful with RC lipo because mis-handled it can burn your house down.

Edit: contact ebikes.ca, em3ev, pingbattery, or Luna and see what they have to offer.

Highxtech said:

I'm going to try another ride tonight. I know lead is old tech but its what it came with. Can anyone recommend a lithium or more reliable pack in the 84-96v 20+ah capacity?

thanks for the advice everyone

Click to expand...

Is that the voltage and capacity that you have now in lead acid batteries? That is a lot of weight !

Any other chemistry will be much lighter. The best counseling for the type of batteries that is best for you, does require to know your current draw. In any case, you will be better assembling your own than buying ready made batteries. I would recommend one series of big cells, making the battery assembly and monitoring easier.

Voltage sag under load is common from Sulfated (worn out Lead Acid batteries).
Allows slow, limited transfer of electrons from electrolyte to lead plates.
Also check for poor connections if sag is nearly immediate under heavy throttle. Heat at any connection or fuse.
Check individual battery voltages after gradual-deep discharge

20Ah Lithium Ion battery will weigh about 20% as much, go 200% as far and last 500-1000% the number of cycles, but, of course will cost much more!

Lead acid batteries might be usable for storage (solar?) or other lower demand application - emergency power through inverter?

They would almost certainly benefit from some type of desulfation. Either a commercial desulfator such as from "Batteryminder", or from a "shock charge" from a higher voltage Flooded Lead Acid charger, as in a car charger. Read-investigate procedure on YouTube or other. Been nearly a decade since I tried it ... Battery must be maintained at reduced temperature and improvement will be gradual and limited! (Test and treat 12V batteries individually, often single battery or batteries deteriorate quicker)
or
Batteries worth $0.30lb at recycler!

Lithium Ion would require 24s x 3.6V = 86.4V minimum 10Ah to match Lead Acid 7 x 12V = 84V 20Ah
Performance and lifespan greatly increased with larger Ah Lithium!
Lithium specific charger required!