replacing a NiMH 24v 14Ah Pack

[panurge]

Hi guys

I'm posting here because an old lady e-bike from a friend that has dead batteries.
The bike has a long alu case that holds up what I think should be a 20s pack made by 20x14000mAh NiMH cells G size (well what I'm sure about is that they are NiMH, 24v, and that they are 20 packed in a 4x5 columns assembly) There is a marker sign that says 14A in the shrink that envelopes the cells.
The batteries are dead (the girl says they are old and finally dead, plus I measured 0.5-0.8 v per cell after 10 seconds of load from a fresh charge made by the original charger, one time, and my hyperion charger the second ). I'm not familiar with NiMH but seems there is no BMS at all, I can see only a 30A fuse inside the case, other than wires....
It is a 6 year old bike and has a big geared hubmotor (BPM size) in the rear wheel (no drag while pedaling) and is rated for 250w24v.

SO I'm Looking for:

- A source for the same G size NiMH 14Ah batteries, cells or the whole pack with the same case or
- the best way/sources to replace them with Lifepo4 or LiIon cells maintaining similar Volt/Amp
(chemistry, cell size, series count, charger, management...) keeping in mind that I'm constrained at the case dimensions that are exactly 4 paired columns of 5 G size cells)...
I know there are same size LiFe Cells but the difference in volts is huge and would need a bms, and a different charger....anyway a good source for these G size LifePO4 is welcome.... :wink:

I will add some pics later.

Thanks

 

[amberwolf]

The bike has a long alu case that holds up what I think should be a 20s pack made by 20x14000mAh NiMH cells G size (well what I'm sure about is that they are NiMH, 24v, and that they are 20 packed in a 4x5 columns assembly) There is a marker sign that says 14A in the shrink that envelopes the cells.

I haven't personally seen G size, only F-size, which are same diameter as D cells but nearly twice as long, IIRC. If you measure the cells and put that up here we can figure out what size they actually are. Without that we can't be sure enough of the case or battery dimensions to help you find replacements.

Does it say 14A or 14Ah? Huge difference. 14A means it is only meant to provide a max of 14A to the motor, and 14Ah means it has a 14Ah capacity.

The batteries are dead (the girl says they are old and finally dead, plus I measured 0.5-0.8 v per cell after 10 seconds of load from a fresh charge made by the original charger, one time, and my hyperion charger the second ). I'm not familiar with NiMH but seems there is no BMS at all, I can see only a 30A fuse inside the case, other than wires....

NiMH and NiCD don't use a BMS normally, just a thermal sensor in the middle of the cell pack somewhere to turn off the charger at a certain point or if the cells overheat for any other reason. Sometimes the thermal sensor (thermistor, usually) is also hooked up to the controller to turn off discharge if they get too hot, but I havent' seen that myself.

It is possible they are actually dead, but it is also possible the charger is not actually charging them fully. How long does it run, and do they get hot near/at end of charge? (they should) Most chargers determine end of charge by when the cells get hot, some do it by when they drop in voltage after a certain voltage is reached, and some (the better kind) do it by both.

- A source for the same G size NiMH 14Ah batteries, cells or the whole pack with the same case or

I don't know of a specific source, but Tenergy might have them, as they have sold a lot of NiMH and related chargers and cells in the past.

- the best way/sources to replace them with Lifepo4 or LiIon cells maintaining similar Volt/Amp
(chemistry, cell size, series count, charger, management...) keeping in mind that I'm constrained at the case dimensions that are exactly 4 paired columns of 5 G size cells)...
I know there are same size LiFe Cells but the difference in volts is huge and would need a bms, and a different charger....anyway a good source for these G size LifePO4 is welcome.... :wink:

Well, if you used Li of some flavor, you wouldn't use the same size cells probably, as even if they are available somewhere you wouldn't end up with the same voltage/capacity if you used the same number of same size cells.

You're probably going to have to compromise on something, either voltage or capacity, to fit within the case.

20 x 1.2V nominal (NIMH) is 24V. Peak voltage is 20 x 1.4V = 28V.

So to get the same thing in LiFePO4 is gonna be... 24V / 3.2V = 7.5 cells. 28V / 3.6V = 7.7 cells. So as long as the bike's electronics will take the extra voltage, you would be best using 8 series LiFePO4 cells (and possibly several parallel strings of 8). Then you'd have 28.8V peak, and 25.6V nominal.

Then for capacity and fitting in the case, you'll have to look around at what cells are out there that will fit in that space, and will give you the best compromise between fit and capacity. If you're lucky you'll find a way to do it that will give you *greater* capacity than before, but probably it will end up less, just by murphy's law.

Don't forget you'll want to fit a balancing BMS in there, too, unless you are going to buy and use a balancing charger for it, *and* a monitoring BMS to provide cell-level LVC for discharge to prevent pack damage from unknowingly riding it to destruction.


Use 3.7V nominal and 4.1 or 4.2V peak for LiCo / LiPo (but I wouldn't use those on someone ELSEs bike, only on my own, especially if htey don't know enough about batteries to do this repair themselves) to figure out how many series cells you'd need.

 

[panurge]

Thanks Amber for your reply....

Indeed they are "f" size (my mistake while writing) anyway I've extracted the cells from the case, and get some pics, you could see with the same marker 24v13Ah but also 14700 somewhere.
The bike owner is a girl that knows nothing about e-bikes, so lipos are out, life in "F" size exists, maybe I could extend the range with more Ah while keeping the same Voltage staying away from murphy's law^_^....but than BMS and New charger are needed. The girl asks for A NiMH replace as first choice....
I'll look at tenergy thanks for the advice...
What I've found in NiMH is more expensive than my expectations...
LiFe seems in the same price range....

 

[ambroseliao]

You'll find what you need here.

http://www.batteryspace.com/fsize---12ah.aspx

Specifically,
http://www.batteryspace.com/Customize-NiMH-Battery-Pack-12V-13Ah-156-Wh-10-x-F-Long-in-two-long.aspx

Ambrose

 

[dnmun]

you should put an ammeter on the charger to see if it is charging. check to see if there is voltage on the output and then put the ammeter in series with the charging current to measure the rate of charge. most likely it is a bad thermistor connection that is stopping it from charging.

 

[panurge]

You'll find what you need here.

http://www.batteryspace.com/fsize---12ah.aspx

Specifically,
http://www.batteryspace.com/Customize-NiMH-Battery-Pack-12V-13Ah-156-Wh-10-x-F-Long-in-two-long.aspx

Ambrose

Thanks Ambrose

I looked at batteryspace as per your advice but they do not ship to Italy and the cost eventually would be quite high (320$ + shipping + VAT +customs charges).

If somebody knows somewhere in the EU or UK where to find them this would be great....

Asked for a quote from Tenergy thru my company...we will see

 

[dogman dan]

Your best bet might be lithium, but not "lipo" aka lico. You might ask Cellman at Em3ev about a custom pack made to the shape you need.

It will be safe, and all she will need to know is plug it in. Your main problem is not finding a battery, but finding a battery that fits that long narrow container.

 

[panurge]

you should put an ammeter on the charger to see if it is charging. check to see if there is voltage on the output and then put the ammeter in series with the charging current to measure the rate of charge. most likely it is a bad thermistor connection that is stopping it from charging.

The charger works good....the Cells are dead....the charger seems pretty solid with fins and a built-in fan cooler, It pushes 3Amps at 24v....
also I've used my Hyperion to see exactly what happens while charging at 1.5A 20s NiMH and it charge the pack very quickly and it shows 1Ah flowed to go from 7v to 28v Ah !!!....
Then The Pack, plugged onto the bike, lights all the status leds like a full charged pack....but in less than a minute, discharging it without load on the wheel, it drops again at the same low voltage....

I'll ask a quote from cellman for a pack plus bms that fits this old slim case and a proper charger....that's a good Idea....

I will post how this replace goes, and also a pic of the motor to see if some of you guys is able (no doubt there) to give it a name and a rate....

Thanks for all the suggestions

 

[dnmun]

can you cut off the plastic sleeve and measure cell voltage of each cell? you may be able to isolate just one or two cells that are bad and then the repair would be cheap enuff to consider.

 

[ambroseliao]

NiMH cells are notorious for building up internal resistance. It sounds like this is the case. There's not much you can do once that happens but to replace the cells.

 

[dnmun]

they may have reversed only one or two cells and if he can find them then he could try replacing just the bad ones and then finally get it to charge up and balance.

 

[docnjoj]

BatterySpace (Tenergy) NiMh were notorious for not delivering amps or Ah even at only 5-10. They were once the only game in town but now.................! I used to get them with tabs and soldered hundreds for my electric gliders and bikes. Gotta be a better way.
otherDoc

 

[999zip999]

Out with the old in with the new. Get a new lifepo4 battery with BMS and charger. This way she can use as a plug and play.
We need to know the amp demand of your controller for the right battery.