Hello All,
I hope this note finds everyone is well & enjoying today!

I have a battery powered lawnmower. The battery pack is labeled as 60 volts 5 Ah. But there is a mystery of how the pack rating is delivered by the cells inside.

I had to open the battery pack after smoke came out while charging. (Problem solved- Was a loose battery tab floating around inside. It must have momentarily shorted two opposite polarity terminals & thankfully acted like a fuse, burning in half before a cell was ruined.)

When I opened the pack's case, I saw that the cells are 2500 mAh. They are Samsung INR18650-25R M. The data sheet shows them to be 2500 mAh with a nominal 3.6 volt rating.

Therefore I expected 32 cells with parallel groups of two cells each for the advertised 5 Ah. I expected that each group of two would be wired in series with the next for 57.6 volts. This isn't what is in the pack.

In reality, there are 26 cells. I could not get in deep enough to see if they are 100% split into pairs for 5 AH without major surgery. The cells I can see are in paralleled groups of two. And groups I can see are in series.

To My Mind Something Isn't Correct:
Twenty-six cells divided by two (for parallel to get 5 Ah) would yield 46.8 volts. (13 x 3.6 = 46.8)
When charged, the pack voltage measures at 62.0 volts. Sixty-two divided by 3.6 equals 17 pair of two cells. That would require 34 cells- But we only have 26 cells.

If I put my brain that the 5 Ah is a marketing scam and the pack is really 2.5 Ah at 62 volts, our 3.6 volt cells in series would give us 93.6 volts.

Also note that there is a circuit board in the pack, but I don't know its purpose. Perhaps there is "voltage magic" happening from the board, but I would not know what the magic could be. One side of the board isn't visible, so I can't seek clues.

Do any of you know how this pack is wired to give 62 (peak after charging) volts at 5 Amp Hours? I apologize, but the cell count and amp hour rating versus cell actual mAh rating does not add up in any formula of which I can find.

Thanks Very Much For Educating!
Paul

 

Sorry for the 2 month delay in responding. Most power tool manufacturers now rate their Li-Ion cells as 4 volts each instead of the proper 3.6 volts. You should actually have 30 cells total (15S2P) to give you 4.0 x 15 = 60 volts and 2.5 x 2 = 5.0 Ah. Fully charged, each cell should give you 4.1 ~ 4.2 volts which is where your 62 volts comes from (4.13 x 15)
HTH
Steve

 

I would hope the circuit board is some form of cell balancing plus undervoltage lockout. But it might just be only undervoltage lockout, preventing discharging the pack any further when any cell pair shows below its minimum rated voltage.

 

Thank You, Steve, for your very good explanation. Now the math all makes perfect sense to me. Since I didn't un-solder the pack to see way inside the mountain, the extra 4 cells must be in there.

Thanks, RPLaJeunesse, for your mention of the balancing & lockout circuits.
I will investigate to find out if it has a lockout circuit so neither the borrower neighbors nor myself don't inadvertently damage the cells by over-discharging. A (careful) bench load test should tell me if the pack stops output voltage at a certain level.
I don't know how to figure out if it has a cell balancing circuit, so I'll have to go on faith & hope.

Thanks Again For Helping & Teaching.

Please Enjoy This Day,
Paul

 

don't know how to figure out if it has a cell balancing circuit, so I'll have to go on faith & hope.

If it has balancing there will be a connection to each junction of two cells in the series, ie, 31 wires in all from the charger to the the batteries.

Bob

 

Just be careful,
you have a lot of energy there, and over the low voltage directive

i.e you have enough voltage to electrocute your self, and enough energy to make a spark that can blind you
even if the exploding cells don't spray you in burning lithium,

 

31 wires in all from the charger to the the batteries.

Bob

If the balancer is built into the pack there will be no need for wires off to the charger. The balancer does need to be able to shunt each cell pair so excess charge current is dissipated as heat rather than overcharge the protected cell pair. With 15 cell pairs only 16 wires are needed, and it all can be in the pack.

 

Thanks Bob & RPlaJeunesse for explaining how to determine if the pack has cell balancing. I can't see deep inside the cell mountain without disassembling, so when replacing the cells becomes necessary, I'll now know for what I'm looking.

Thank You Andrewmm for the very welcome reminder about electrical safety!
Electricity and I have a Mutual Respect Pact. I respect it's power and it respects my health. My Slogan: "Electricity is a good slave, but a very bad master.".

By coincidence, I am an industrial electrician, former high tension line worker (too cold for me). My current work takes me up to 13,200 vac. I was also a safety director as well as a safety educator in the electrical field. (Both property & personal safety, such as is discussed in NFPA 70E)

Very many people have no safety concerns & no respect for the power of direct current.
Quite often I've heard people make comments along the lines of "It's DC. DC can't hurt you.". I politely, but firmly, instruct otherwise. I think they learn this from stuff they've heard over the years about Thomas Edison promoting DC because it was "safe". He had several (very biased) demonstrations of the dangers of AC versus the safety of DC which are shown in documentaries & taught as fact in some high schools.

Concerning the potential energy of direct current, I often use the arc characteristics of DC as one example. It provides a much stronger and deeper penetrating arc than alternating current of the same voltage- hence the popularity of DC SMAW welding & the reason contact points on contactors powering a DC load pit deeper than their AC counterpart. DC will "jump" much farther in air, as well as provide a far more penetrating & more damaging burn to skin. Both AC and DC are powerful and must be respected.

Thanks Again for your safety reminder!

And Thanks! to all of you for helping me learn about this battery pack. It's lead to much more studying.

Paul
Any Day One Learns Something New Is A Very Good Day!