DC to DC Safely

[ElFotoGuy]

Hello all,

I just finished setting up a complete system from signature solar ((2) 5kwh GroWatt inverters, (3) 48v EG4 batteries). I also just converted a 2001 48v Club Car to LiFePo4 batteries (100ah total).

I would like to be able to connect the batteries from my cart directly to my solar batteries, so I can utilize the full potential of both sets of batteries. I'm concerned about the massive current in-rush when connecting the two banks together. I need something to limit the current, but that extends beyond my knowledge. Right now I'm limited to an ac charger to charge the cart and if I would want to send power back to my solar bank I would need to hook up a separate inverter to run my 18a charger. I don't like this option because of efficiency losses and the time it would take to transfer the power in the cart back into my solar battery bank.

How can I set up a back and forth DC to DC charging system safely and what components do I need?

 

[Spunky]

If you had an old spare charge controller, you could look into using your EG4s as the solar panel and your golf cart at the batt?

I find DC to DC converters for sale, but those are like trickle charges for your use.

 

[ElFotoGuy]

I hadn't considered that. I suppose I would need to add a step-up converter in order to trick the charger to increase the charging amperage? I do have a small charge controller I might experiment with. I would also want to get a switch that should allow me to charge in either direction.

 

[ElFotoGuy]

After some consideration I think I will use my old charge controller for DC to DC charging.

Now I need a switch if some kind to be able to switch from charging with the battery bank to the cart, and the other way around - from the cart to the bank. I don't think a generator transfer switch will work since it only has one output. If I'm thinking correctly I would need two inputs and two outputs since I will be switching back and forth between opposite inputs and outputs.

 

[Spunky]

Here is an active thread about this exact thing, with the good and bad.

 

[MrThisIsME]

If you had an old spare charge controller, you could look into using your EG4s as the solar panel and your golf cart at the batt?

Most charge controllers need to see a voltage significantly higher than the output. At least that I am aware of.

 

[BradCagle]

A solar charge controller may not work so great (if at all) with the same nominal voltage on both sides. Can always give it a shot.

What really needs to happen here is:
First: A device needs to equalize the voltage of both banks, this would be a bidirectional transfer of power.
Second: Once equalized, the device should trigger a relay/contractor to create a high current path between the two battery banks.

 

[Sverige]

A solar charge controller may not work so great (if at all) with the same nominal voltage on both sides. Can always give it a shot.

What really needs to happen here is:
First: A device needs to equalize the voltage of both banks, this would be a bidirectional transfer of power.
Second: Once equalized, the device should trigger a relay/contractor to create a high current path between the two battery banks.

Where do you find a bidirectional DC converter with a fixed output which would allow for two such devices to be connected on a transfer bus between a pair of batteries?

I‘ve been wondering if there’s a way to do this to allow a 4S LFP battery to be charged in parallel with a lead acid bank such that both batteries share the charge or load all the time, instead of the conventional approach of putting them in parallel with no converters and having the lead acid bank only discharge once the LFP got down to 10% or so.

 

[ElFotoGuy]

Most charge controllers need to see a voltage significantly higher than the output. At least that I am aware of.

That's what I'm afraid of right now. However, my main bank and the carts batteries have enough capacity that there shouldn't be much voltage sag. The problem is that LFP batteries voltage curve is so linear that I'm not sure that I will get much usable power. My guess is the receiving battery will quit charging (significantly) at 30% even though the sending battery is still at 70%. Probably a step up converter would be needed. Then I would be switching four conductors instead of two! ??

 

[BradCagle]

Where do you find a bidirectional DC converter with a fixed output which would allow for two such devices to be connected on a transfer bus between a pair of batteries?

Exactly. Probably would have to develop one.

I know of a way to do it, and it's much like how a capacitive active balancer works

Watch my video of how active balancers work, and I bet you'll see what I'm thinking:

Now don't think I'm suggesting to use an active balancer to do this, I'm just showing the principal is the same

 

[Sverige]

Exactly. Probably would have to develop one.

I know of a way to do it, and it's much like how a capacitive active balancer works

Watch my video of how active balancers work, and I bet you'll see what I'm thinking:

Now don't think I'm suggesting to use an active balancer to do this, I'm just showing the principal is the same

Yes I see your point. You could use switched capacitors to transfer the power in both directions, but would need some pretty big capacitors if you’re not to inhibit the system performance by too much.

It’s an interesting possibility and I’m intrigued by the opportunity to use lead acid and lithium batteries equally in a system, rather than essentially one after another. Once an equipotential power transfer bus is established you could even get more sophisticated with the control and decouple the LFP battery so it is not drained overnight by the lead acid self discharge.

I think this may be how the new Seplos Polka battery is designed to work, but details of the operation of that are scant at this stage.

 

[BradCagle]

Yes I see your point. You could use switched capacitors to transfer the power in both directions, but would need some pretty big capacitors if you’re not to inhibit the system performance by too much.

Yeah, but you only need this to perform the equalization step. Once equalized, the system would close a high current contactor/relay fully connecting the battery in parallel to the main bank.

So you have some time before the battery comes on line.