Use golf cart batteries in forklift?

[georgia088]

Wow ok. I’m guessing most forklifts are driving them harder than the recommended 20% discharge….

So, if I have to use a capacitor, to prevent the inrush, is the following picture how I would wire it in? What size capacitor would you think is needed?

[TheCartBoy]

The capacitor would be in parallel with the battery, + to +, - to -. But you should charge it initially through a resistor, otherwise the BMS will probably think there's a short circuit.

I would probably go with a 100v rated cap to ensure you never risk overcharging it (BANG!).
I see 100v 22000uf capacitors available, gut feel that would be suitable.

But here's some math to help, this is just a wild guess at what the in rush looks like on your pump motor, measuring it would help with sizing the capacitor.

To calculate the capacitor size needed to support a 50 amp inrush current at 48 volts, we'll use the following formula:

C = I * t / V

Where:

C is the capacitance in farads
I is the inrush current in amperes
t is the desired discharge time in seconds
V is the voltage in volts
Assumptions:

We'll assume a desired discharge time of 1 second for this calculation.
Calculation:

C = 50 amps * 1 second / 48 volts
C ≈ 1.04 farads

[georgia088]

While I process that math, could I measure it with a DVM clamp meter? I didn't know if it would catch the suddens spike of current with it?

[georgia088]

Also, if your math is stating approximately 1.04 farads at 48v, why do you think a 100v 22,000 uF would suffice? isn't 22000 uF much smaller that 1 farad? I know nothing about this, as I'm sure you can see.

[TheCartBoy]

If your meter has a max/peak mode that would give you one data points. The in rush is going to be fast maybe only a few milliseconds. What did you increase the time limit on the BMS to? That might actually give us a better idea of much current and how long.

The real measurement tool would be a current meter and oscilloscope to capture the full power up sequence.

Also I'm not saying you need to add the capacitor, when you have the 3 batteries in parallel it might be fine. But we can continue the thought process 🤓

[TheCartBoy]

Originally Posted by georgia088

Also, if your math is stating approximately 1.04 farads at 48v, why do you think a 100v 22,000 uF would suffice? isn't 22000 uF much smaller that 1 farad? I know nothing about this, as I'm sure you can see.

Yes it's much smaller. The math is for completely discharging the capacitor, the problem we are trying to solve here is just to prevent a large current spike as seen by the BMS, we aren't expecting to run the motor off the stored energy in the capacitor alone.
The 22000uF capacitor looks like a mini soda can, I'd think some where between that and a beer can sized capacitor would be sufficient.

[georgia088]

The default setting for short circuit protection is 1500μs at 10000μs it quit tripping the bms. However, it did trip again later after doing a lot of moving around and playing. So, I am guessing it is still a little low to be reliable. By the way, I am really hoping to only use 2 batteries in parallel. They are rated at 3c for thirty seconds at 105 Ah so around 315 amps for 30 seconds. 6Cfor 3 seconds ( I think).

It would be double those numbers with two in parallel obviously and the two ac motor controllers are rated at 350 amps a piece.

I guess there could be a situation where if both drive motors are at their peak current and the pump motor pulls large current, I could trip the bms. However, that seems fairly unlikely, and the BMS should protect it from this.

[TheCartBoy]

Ok so needs at least 10 milliseconds of energy, if you can get a read from the clamp meter that would be the next piece.

[georgia088]

I didn't check the amperage draw on startup, but I did put two batteries in parallel and use it for about 20-30 minutes yesterday. I lowered the short circuit protection down to 8000 milliseconds on both packs. It never did trip. It seemed to work perfectly. I may lower the setting down some more to see how low it will go before tripping.