36v battery overcharge to 41v

Thamosy

Pedelecer
Jan 14, 2013
55
1
Hello. I recently ran my Li-ion bottle type battery (36v 9Ah) almost completely flat (red winking light) and when I came to recharge it was surprised the charging time was relatively short as compared with a similar event a few months previously. The battery is now showing 41v.

Not realising what had happened I used the battery - to find that it cut out when under load (I now guess that was the controller doing its job) and it also ran down very quickly.

Is there any way of retrieving what I suspect is a now useless battery? If not it'll be an expensive pity because I've only had this battery just over a year and feel I've thrown in excess of £200 down the drain.

I knew little about battery technology when I bought this "cheapo" (on ebay) and believe it is NOT a LiFepo4 one. Can anyone advise?
 

flecc

Member
Oct 25, 2006
52,763
30,349
There's nothing wrong with that voltage, the fully charged voltage of a nominally 36 volt lithium battery is 41 volts, and the cutoff is set for about 31/32 volts. The fault is not yours, it's only that the voltage is collapsing under load, possibly due to failing cells in the battery. Does the battery have a manufacturer's branding or can you give any more details about it?
 

Thamosy

Pedelecer
Jan 14, 2013
55
1
It's a cylindrical bottle style battery, 36v 9Ah which was described as 'lithium' and not knowing much about batteries at the time all I wanted was a spare that was light in weight. It will (when good) give me around 8 to 9 miles and I've been well pleased. After charging in the past it's never given me such a high voltage - around 38v has been usual and it's never taken such a short time to reach full charge before. I'll be taking it for a spin tomorrow and I'll see how it goes then, but I wish battery advertisers would be more specific about the chemistry of their batteries. I checked an identical on one of Chinese web sites and believe this to be LiMnO2, but just giving 'Lithium polymer' as a description isn't much help when I started reading up a bit more.

Maybe it's a good job I kept my old lead-acid battery! (Only joking!)
 

billadie

Esteemed Pedelecer
Apr 27, 2010
289
48
Tewkesbury
I would expect to get about 25 miles out of a fully charged 36v 9Ah. If you've only been getting 8-9 miles from new I think you should have a word with the supplier. If they are UK based don't be fobbed off with excuses, you are entitled to a working replacement or refund. If they are overseas your stuffed I'm afraid.
 

flecc

Member
Oct 25, 2006
52,763
30,349
38 volts is definitely too low for a full charge in a 36 volt* lithium manganese battery, and as Billadie says, you've been very short on mileage. It's a mystery why it should suddenly achieve a higher, more correct, voltage now, but from that range it's certain that its never been performing as it should.

*A nominal 36 volt lithium manganese battery uses 10 cells series, and any on-line article on these will confirm that full charge should be 41/42 volts, as will any experienced user of these in here.
 

trex

Esteemed Pedelecer
May 15, 2011
7,703
2,671
Your battery may have a faulty cell. The simplest way to find out which one it is, you'll need to ride the bike until the battery is quite flat then check the voltage of each cell. Good cells show a voltage above 3.2V, bad cells below 3V.
 
D

Deleted member 4366

Guest
Most of the bottle batteries use lithiun manganese cobalt chemistry. Fully charged, they should be 42v. 38v is miles too low hot off the charger, but is what you'd expect to see when riding the bike.

Your battery has gone out of balance, which has the effect of low capacity. You need to re-balance it. First, try charging it up snd leaving it on charge for a long time afterwards. If that doesn't restore its capacity, you need to take off the end-caps, slide out the core and measure the cell voltages. The battery comprises 40 cells in 10 strings of 4. Each 4 are connected to the BMS, which measures the voltage and switches off power or charging when they get to certain high or low values. When unbalanced, the BMS switches off too early. Once you'ved measured the cell voltages yourself, you can rebalance the cells with a phone charger or similar. We'll explain how to do that when you publish the vell voltages. You need a multimeter to measure the cells. Ask if you need any help with this and we'll give more details.


Sent from my GT-N7100 using Tapatalk 2
 
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Thamosy

Pedelecer
Jan 14, 2013
55
1
Thanks so much for your comments flecc, billadie and d8veh. They were very useful. I think I’ve now solved the problem, but first I’d better give you the “egg-on-face” result, for which I feel a bit embarrassed. It’s revealed a couple of things that might be useful though.

First, I made a mistake about the ‘usual’ full charge being 38V. That’s a figure I remember from lead-acid days and when checking my available batteries after your responses, the not-so-old and still functioning lead acid still gives me 38V, but my second LiPo4 battery is, as you suggest it should be, 41.5V. So, I’m happy about that.

Then I STARTED to take the advice about sliding off the bottle end-caps to measure individual cell voltages, but got no further than realising the lower end cap was already departing the body of the battery! There was a gap of around 1 cm! Also the fuse, which had unscrewed to a good 0.5 cm! With it in this state I’m not surprised the power had started cutting out!

The explanation is simple and it’s that I rarely ride on anything that might be considered a smooth surface (pretty well all my outings are off-road) so vibration has done this. It was a useful lesson, kicking me into checking whether anything else had worked loose. Nothing critical had, but I’d lost a nut from a light fitting and I needed a spanner on a few other bolts. It was a useful warning.

As to the problem of the shortened charging time and loss of ‘distance run’, I should explain that I’m a spinal patient and rely almost exclusively on power from the motor. For me, e-biking is a million times better than sliced bread in that I’m still able to enjoy bike runs. With 2 batteries I’m good for about 15 to 20 miles in the Peak District (where I live) so the short ‘usual runtime’ is expected because I’m both battery driven and have some pretty big hills to get up (and without the ability to get off and push)!

You might say none of this explains the shortened time to full charge, but I’ve been doing a bit of reading and came across this article – which explains what is happening. Over the weekend I checked it out.

This is the link and the relevant bit.


Charging Lithium-Ion Batteries


Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because high voltages stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. Since the consumer market promotes maximum runtime, these chargers go for maximum capacity rather than extended service life.

It's worth looking at the diagram in that article.

When charging the battery on the occasion that caused me to panic, I’d not done it in one continuous charge, but because I’d arrived home late had put only around 30 min charge into it (I considered it better to do that than leave it flat(ish) for perhaps some time). When I then completed the charging later it was “full” in a very much shorter period of time (by nearly 1 hour)

So, over the weekend I’ve had a couple of runs to flatten the battery (well, it was glorious weather) then with one re-charge I did it continuously, whereas the other re-charge I did in two stages. The same thing happened again with continuous charge taking around 50 min longer than one in which I paused the charging after the first hour and then completed it 2 hours later.

It all now makes sense to me (I hope it does to you too) and my only problem is now the one of choice, to opt for preserving the battery with an interrupted charging and have less runtime, or to opt for maximum runtime and so reduce battery life.

But – big BUT – as you’ve implied that it’s easy(ish) to replace individual cells in a dying battery, then for me I’m going to opt for the maximum runtime.

Next question, where’s a recommended place to buy the individual battery cells?
 
D

Deleted member 4366

Guest
You shouldn't take too much notice of articles like the one you read. They're often not relevant to what you have. Your charger is designed to charge your battery in the optimum way. You shouldn't run your battery flat if you can avoid it. You should just use your bike and put it on charge when you get home until the light goes green.
 

Thamosy

Pedelecer
Jan 14, 2013
55
1
The problem is that with interrupted charging the green light does go out and the battery won't accept anymore charge. I don't yet know whether that's just with this particular battery, but I'm loath to leave it on charge and not switch off within a few minutes of that green light.
 

tushingham2

Pedelecer
May 9, 2020
119
3
Hi Very interesting i have a lithium battery quite new 36v full charge shows 42v i go less than 1 mile it shut down unplug it plug in again and okay for another mile. I replaced the BMS but no better, so i will see what i get testing each cell. I checked all connections and the fuse all okay

Thanks Ron
 

PC2017

Esteemed Pedelecer
Sep 19, 2017
1,300
327
Scunthorpe
Thanks Ron
Try starting a new thread the last post above yours was dated 2013. Do your testing and post a new thread with YOUR original question and the test results. Best of luck
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
9,822
3,985
Basildon
Hi Very interesting i have a lithium battery quite new 36v full charge shows 42v i go less than 1 mile it shut down unplug it plug in again and okay for another mile. I replaced the BMS but no better, so i will see what i get testing each cell. I checked all connections and the fuse all okay

Thanks Ron
I'm going to guess that your battery is under-rated for what you need. Start a new thread and provide all details about your kit.