Golf Cart - Reducing 48v to 36v at 200Amps

[ThermallyFrigid]

Hello!

I have a 36v EZGO golf cart and a bunch of 100Ah 48v LiFePO4 batteries.

I'd like to be able to use these with my 36v EZGO golf cart but I cannot find a Buck converter to do this.
I think the peak possible load would be around 200Amps at 36v

I "could" convert the cart to 48v but the costs to do that and work involved are a bit high and I don't see any need for converting it since I'm fine with the power and speed at 36v.

Any suggestions?
Thanks

[jbb]

I have 3 thoughts
1) A 36V 200A buck converter is pretty big. Not sure if you could find one off the shelf
2) maybe some parts of your golf cart would be OK with a 48 V input already; converting the cart might not be a big deal? Maybe?
3) are you talking about a 48 V monolithic battery pack, or a cells+BMS pack? Just using fewer cells to make a 36 V battery seems like a good approach (if possible)

[ThermallyFrigid]

Hello jbb,
They are monolithic 48v packs that I cannot disassemble.

[moffy]

If you don't need 200A all the time, you might get away with say a 20A. 48V to 36V converter feeding a single 36V battery which is fed from the 48V pack. It would put a lot of stress on the 36V battery and would be dependent upon the current duty cycle being well below the maximum required.

[PCB.Wiz]

Any suggestions?

Aliexpress have buck converters like this, seems quite low cost for 50A rating.

https://www.aliexpress.com/item/1005006318553257.html

that could be used (as above) to charge / float the 36V system you have now.
I'm not sure how they manage parallel current sharing.

[Siwastaja]

Motor controller IS the buck converter. First see the specs of the controller if it could work with 48V pack directly. If the original motor controller cannot handle the maximum voltage of your pack, then just upgrade to higher voltage motor controller. A separate buck converter is significantly larger and more expensive as it needs a huge inductor and output capacitors, the roles of which are already provided by the motor.

[langwadt]

Motor controller IS the buck converter. First see the specs of the controller if it could work with 48V pack directly. If the original motor controller cannot handle the maximum voltage of your pack, then just upgrade to higher voltage motor controller. A separate buck converter is significantly larger and more expensive as it needs a huge inductor and output capacitors, the roles of which are already provided by the motor.

yeh if something like this fits, https://www.ebay.co.uk/itm/386262614007
that is probably cheaper than trying to build a 200A buck

[jpanhalt]

According to the theory, maybe the solution is just to limit the controller to roughly 75% full scale: https://www.4qdtec.com/pwm-01.html

This [20 kHz] is far too fast for the poor old motor to even realise it is being switched on and off: it thinks it is being fed from a pure d.c. voltage.

Have you tried it?

[Siwastaja]

According to the theory, maybe the solution is just to limit the controller to roughly 75% full scale

The only question is, are the MOSFETs and capacitors rated for the higher voltage. Therefore a glance at the specifications of the installed controller is necessary. It's highly possible it already supports 48V(nominal) pack because manufacturers don't make a gazillion of different models, and 48V is a pretty common pack voltage, too.

[jpanhalt]

I agree it was an assumption.  The power mosfets I have used were all rated at 60V VDS or more.

[Siwastaja]

The power mosfets I have used were all rated at 60V VDS or more.

... which are not suitable for a 48V battery pack. If lead acid, it can be at up to 15*4 = 60V while in charger; or even if you have interlock, easily 13.5V*4=54V right after charge. For 48V LFP pack, that would be 3.65V*16 = 58.4V at the end of charge.

Remember there is always ringing of parasitics (layout inductance resonating with Cds). Avalanche rated MOSFETs might be able to take some... but it's iffy. Unless you do extraordinarily good work with layout & snubbing, you should leave some 15-40% margin for Vds_max (and of course verify with a high bandwidth oscilloscope, with maximum load current). For a 48V pack, 80V or 100V MOSFETs would be normally used.

[pienari]

I think controller and motor can handle 48v battery.
Like 36v e-bike controller caps are rated 60v and works fine with 48v.
Have you tested?

[jpanhalt]

The author is considering using LiFePO4 batteries, nominally 3.2 volts.  So, 15 cells = 48V.  Besides, he asked for alternatives, and suggestions for using a buck converter did not seem practical. 

I have no idea what a 48-volt controller alone would cost, but that is probably cheaper than "converting" the whole cart to 48 V, which I assumed meant changing the motor too.

[Siwastaja]

Motor definitely doesn't need to be replaced. Only controller (and obviously battery charger).