Bad battery - need help with diagnosis

[CanyonD]

I left a spare Radrover battery sit for a long time and when I went to charge it, the charger did not recognize it. I measured only 20V at the output terminals that connect the battery to the motor. Since this is a 48 V battery, I figured it was below the minimum voltage level that the charger requires to charge.

To revive the battery, I hooked up three solar panels with an open circuit voltage of 56 V to the output ports. The current was modest, only about 1.6 A max. After a few hours, the output port only registered about 24V. Long story short, I opened the battery, and measured voltage of the different sections of the battery bank and noticed the max differential was 52V, which is a fully charged 48V battery.

The battery bank is constructed of (52) Samsung 35E 18650 cells. They have only had about 10 cycles, so I would like to fix this problem and not dispose of valuable good lithium cells. I think the cells are in good shape, but if the battery has an internal voltage of 52V, why am I only getting 24V on the output terminals? Bad control board?

Strangely, when I measure voltage at the charging port, I get 26V.

Any advice?

 

[CanyonD]

UPDATE: I discovered the problem. The battery was in "lock" mode. I turned the key and the voltage was back to normal. False alarm, but I got to totally disassemble the battery to see the inner workings and configuration. Fun project

 

[AguassissiM]

@CanyonD

Welcome to the forum.

Hope you have taken few pictures of the inside of the battery, always good to have those....
I`m just glad that you still have a working spare battery.

 

[AHicks]

I'm glad you didn't write it off without doing a little poking around first!

 

[harryS]

Be careful in the future when putting a charge on a battery's output ports, as there is usually no over voltage protection on the output. It is only in the charge port. You would be better off using the charger as it's limited to 54.6V, the max allowed voltage for a 48V pack. But if the pack was in lock mode, then probably nothing was connected to the cells.

This diagram is for a 36V battery, but is the same for a 48V pack. You can see that the negative end of the internal cells is connected to the respective negative leads of the output and charge plugs by transistor switches controlled by the battery management system (BMS).


When the circuitry turns these transistors off, there should be no voltage, unless the transistors leak a little bit. Usually, the leakage is so low that if you turn on the bike with the battery off, the small load will drop the voltage to zero.







.

 

[CanyonD]

harryS, I understand about the challenges of overcharging via the output port, and I measured cell voltage and all were at 4.13V. The max voltage is 4.2, so my charging through the output ports did no harm from what I can tell.

AguassissiM, Here is a photo inside the battery, but I only took 3 photos. That said, it only takes a few minutes to disassemble to take more..

 

[AguassissiM]

@CanyonD

That is awesome, thank you for sharing.

 

[CanyonD]

Update: I have gone through discharge/charge cycle with the battery that I disassembled, and everything looks good. The voltage after charging was 54.28 V. This would equate to 4.17V per cell. Since I likely did no charging with my efforts above, this would infer that the samsung cells lost very little power over the year that the battery sat idle.. When I turn the key to the lock position, output terminal voltage drops to 24V.

My current opinion, a fully charged Rad Power Bike battery can likely sit for extended periods without being charged and not drop much in voltage.

 

[AHicks]

Re: your opinion, that's pretty much everyone's take using this lithium based battery chemistry. Par for the course....

It's one of the reasons a lot of guys don't charge fully, right up to max. If they know they aren't going to loose anything while sitting between uses, and they don't need it to make the normal distance they run most often, theory has it they can make their battery last longer by not fully charging. Being "battery friendly" you might say. Myself, I think the jury is still out on that practice, but many "experts" swear it's true!