Help! Newbie to battery packs and BMS could do with some advice please!

[MobilityBoy]

Hello all.

I've been reading quite a bit about Lithium-ion battery packs and their associated BMS / Balancing PCB circuits.

I've recently inherited a mobility scooter (i know it's not an e-bike, but i'm assuming the principles behind the battery packs and the BMS boards are the same), which has what appears to be a dodgy battery.

I'm not ashamed to admit, i know very little about battery packs, and even less about the BMS and the various options available when selecting one, hence why i'm here really.

While I might not know much about these packs *yet*, and much of the battery / BMS terminology eludes me, but i'm usually a fast learner and not particularly thick!

Hoping someone can help (please be gentle!) me sort out the correct BMS balancing circuit for this battery pack i've got.

This is what was happening:

The mobility scooter would run for a very short time (2 X 150W motors), and literally after a couple of minutes, stop dead. The battery meter showing completely flat batteries (has green for fully charged, yellow for needs recharging and red for completely flat indicator lights).

The battery pack is Lithium-Ion, 24V 15Ah, and comprises 49 (7 x 7 rows) of 18650 batteries, with Ni strips connecting the rows. The pack is showing 25V using my multimeter right now.

I discovered the BMS circuit board and thought this could have gone to the great PCB place in the sky...i noticed that when pressure was applied to an area of the BMS board, the voltage reading would drop considerably, so i concluded the board had or was nearly dead...certainly faulty.

I need to buy another BMS (with balancing so i've read) but am unsure which one i would require, as i'm also unsure of how many 'S' and 'P' (serial and Parallel connections / groups) i have in my battery.

A picture speaks a thousand words, so here are some photos of the battery and currently attached BMS (which is not available any more apparently)...


And the other side of the battery pack....



And the BMS board...



I'm assuming this battery pack is what is known as a 7S and a certain number 'P', but i'm unsure of this...if someone could clarify for me, so i don't buy the wrong BMS board, or better still, if someone could recommend a BMS balancing board i would be grateful.

Thanks,

MobilityBoy.

 

[soundwave]

how old is it as sounds like the cells are at the end of there life and there not 18650 cells as way to fat.

 

[Nealh]

Your battery is 7s 7p, 7 cell groups in series with 7 cells per parallel group.
The Red cell ring is the V+ and the Blue un- ringed is V-.
In pic#1 the Black thick wire from cell to BMS is connected to cell group B- /0v, the next cell group to it (Red) is #2, and then the next #4 and the end #6.
On the opposite side of the battery pack pic # 2, the two thick RED wires are connected to cell #7 V+, then proceeding that cell groups you have are # 5 , # 3 & #1 respectively.

 

[Nealh]

Hello all.




This is what was happening:

The mobility scooter would run for a very short time (2 X 150W motors), and literally after a couple of minutes, stop dead. The battery meter showing completely flat batteries (has green for fully charged, yellow for needs recharging and red for completely flat indicator lights.

MobilityBoy.

This describes typically the end of life of a battery pack and is due for recycling,
as SW has said it very much does indicate you need a new battery and not just a BMS.
The BMS deals with the charge and discharge of the cells groups and battery pack as a whole.

Though the battery may well charge fully it doesn't mean it is well, once fully charged if you are able to connect up a voltage meter temporary to the battery so you can see the voltage reading, ride it with power and watch the voltage reading. You should see it sag (drop rapidly ) and then collapse.

 

[soundwave]

This describes typically the end of life of a battery pack and is due for recycling,
as SW has said it very much does indicate you need a new battery and not just a BMS.
The BMS deals with the charge and discharge of the cells groups and battery pack as a whole.

Though the battery may well charge fully it doesn't mean it is well, once fully charged if you are able to connect up a voltage meter temporary to the battery so you can see the voltage reading, ride it with power and watch the voltage reading. You should see it sag (drop rapidly ) and then collapse.

that size of batt was used in 1998 so am amazed it still has any charge at all.

no bms in those days

 

[Nealh]

I doubt you will be able to buy a 7s 7p battery easily unless from a mobility shop (which will likely mean 'HOW MUCH') or you contact Jimmy at ebikebatteries.co.uk and he will replace the cells a lot cheaper but still expect to see a £300 - £350 cost, for that though you will get some of the best cells available.

 

[soundwave]

cheaper option if they will fit but you will need to find a way to charge them but 2 gives you 24v when linked together as this is what my mums uses.

DAYS MEDICAL AIDS (DMA) TINTO S534 MOBILITY BATTERIES X 2 - VAT FREE (2x12-12) | eBay

This declaration has to give enough information to show that you qualify. If you can't sign the written declaration yourself, the signature of your parent, guardian, doctor or another responsible person is acceptable on the declaration.

www.ebay.co.uk

 

[Nealh]

The 25v currently shown is the resting voltage, fully charged you should see 29.4v and empty will be about 21v. Once suspects the 25v will collapse once a load is asked of it.
As you are familiar with voltage reading do as I mentioned and fully charge, take a reading and let us know what it is. Then rig up the meter so you can see what happens to the voltage when a load is asked, take it out and watch what happens to the voltage. One suspects you will see massive voltage sag and then it collapses so the BMS will cut out as the voltage is too low, once the load demand is released you will see the voltage climb up again. This is a sign of end of life for the battery.

 

[Nealh]

Aother option I would consider is a nice lifepo4 battery they will have twice the life of lion and about 6x that of SLA. Depends on the current demand of the mob scooter.

Ultramax LI10-24, 24v 10Ah (240Wh, 40A rate) LiFePO4 Battery - Replace SLA 24V 10Ah with 4 times cycle life, lighter weight, Charger Included - LiFePO4 Batteries - Lithium Phosphate LiFePO4 (batterymasters.co.uk)

 

[vfr400]

Can you let us know what designation is written on the cells?

 

[MobilityBoy]

how old is it as sounds like the cells are at the end of there life and there not 18650 cells as way to fat.

Hi Soundwave,

Thanks for your replies.

They definitely are 18650 cells, i have had a fair amount of experience with this type of cell, but in vaping circles rather than battery packs. They look a bit fatter due to the angle of my poor photography i suspect.

I did worry the cells, or at least some of them might be at fault, but figured buying a cheapish BMS board and trying it would be a better first option than buying a set of 49 18650's and rebuilding the whole thing, costing less than an off the shelf battery pack, but still quite pricey all the same...besides, if i buy the BMS and the problem is due to the cells, at least i could still make use of the BMS if i have to go and buy a set of new cells from China or somewhere.

Cheers.

 

[vfr400]

For any battery related problems, you start by fully charging, then check the pack voltage hot off the charger. Anything significantly less than than 29.4v, you check the individual cell voltages. Next you run the battery until you get the problem and recheck the cell voltages. It also helps a lot to can get a real-time voltage measurement of the pack while using it to see how much sag there is and what happens immediately prior to and at the time of cut-off. The last thing you should do is start fiddling with the BMS. The measurements will tell you what's wrong and what you have to fix.

 

[MobilityBoy]

Your battery is 7s 7p, 7 cell groups in series with 7 cells per parallel group.
The Red cell ring is the V+ and the Blue un- ringed is V-.
In pic#1 the Black thick wire from cell to BMS is connected to cell group B- /0v, the next cell group to it (Red) is #2, and then the next #4 and the end #6.
On the opposite side of the battery pack pic # 2, the two thick RED wires are connected to cell #7 V+, then proceeding that cell groups you have are # 5 , # 3 & #1 respectively.

That's exactly the information i was after Nealh, thanks very much for your help...hadn't realised it was a 7S and a 7P configuration, but now you've said it, it makes sense since there are 7S groups and the battery pack total Ah is 15Ah...(7 X 2100 mah (ish) = ~15Ah.

Cheers

 

[MobilityBoy]

This describes typically the end of life of a battery pack and is due for recycling,
as SW has said it very much does indicate you need a new battery and not just a BMS.
The BMS deals with the charge and discharge of the cells groups and battery pack as a whole.

Though the battery may well charge fully it doesn't mean it is well, once fully charged if you are able to connect up a voltage meter temporary to the battery so you can see the voltage reading, ride it with power and watch the voltage reading. You should see it sag (drop rapidly ) and then collapse.

Starting to think this is indeed the case Nealh...can't afford a new battery, so will have to either sell the scooter as is (no battery) or take your advice about looking into a Lifepo4 battery pack, since they are coming in around half the usual cost of a Lithium Ion pack.
Cheers.

 

[MobilityBoy]

Can you let us know what designation is written on the cells?

I did look for this info myself mate, unfortunately, there is nothing printed on the individual cells at all, that i could see anyway.

 

[MobilityBoy]

cheaper option if they will fit but you will need to find a way to charge them but 2 gives you 24v when linked together as this is what my mums uses.

DAYS MEDICAL AIDS (DMA) TINTO S534 MOBILITY BATTERIES X 2 - VAT FREE (2x12-12) | eBay

This declaration has to give enough information to show that you qualify. If you can't sign the written declaration yourself, the signature of your parent, guardian, doctor or another responsible person is acceptable on the declaration.

www.ebay.co.uk

It's another option to think about soundwave, thanks for the suggestion. It would certainly be the cheapest option, but it's getting the right physical sizes that will fit into the existing battery pack box that will be the problem i reckon...still, something to think about though!
Cheers.

 

[MobilityBoy]

I doubt you will be able to buy a 7s 7p battery easily unless from a mobility shop (which will likely mean 'HOW MUCH') or you contact Jimmy at ebikebatteries.co.uk and he will replace the cells a lot cheaper but still expect to see a £300 - £350 cost, for that though you will get some of the best cells available.

£300 - £350...Ouch.

 

[Nealh]

The lifepo4 option for a mobility scooter is no brainer imv, unlike an ebike the heavier battery weight ( approx. 30%) isn't noticeable on such a vehicle. Once ebikes all used sla, lifepo4, nicad & nimh all varying in weight but them lithium made things a lot lighter.
SLA was always the cheap option, the weight impractical but workable and the issue with SLA is power and performance. The Lifepo4 though a little more expensive will last far longer and have better power and range delivery then SLA and will come with a new charger as the charging voltage is different.

 

[MobilityBoy]

The lifepo4 option for a mobility scooter is no brainer imv, unlike an ebike the heavier battery weight ( approx. 30%) isn't noticeable on such a vehicle. Once ebikes all used sla, lifepo4, nicad & nimh all varying in weight but them lithium made things a lot lighter.
SLA was always the cheap option, the weight impractical but workable and the issue with SLA is power and performance. The Lifepo4 though a little more expensive will last far longer and have better power and range delivery then SLA and will come with a new charger as the charging voltage is different.

Agreed Nealh.

If i do go the new battery route, i think i'll take your advice and go for a Lifepo4 of roughly the same Ah rating...at about half the cost, it would be madness not to IMO.

Thanks very much for your help, it's very kind of you all.

Cheers.

 

[danielrlee]

One more thing to note - mobility scooters & electric wheelchairs often require a certain amount of 'ballast' in order to provide stability. In these cases, lead acid (or LiFePo4) makes perfect sense. Electric bikes only suffer with unnecessary mass and benefit from modern batteries with the maximum possible energy and power density in order to keep weight to a minimum.