DC/DC Converter vs Second Battery

[puddleglum]

Hi guys, I've been trying to think of an easy way to ease the load on my starting battery and still EOC with lights on. Presently, I'm not looking at an Alt. cut off switch, but that might follow If this works well. I've found some threads on this, but not quite what I'm thinking of. Most seem to end up with going to a deep cycle battery. I don't want to do that. It costs money (I'm cheap), adds weight and takes up valuable space.
I currently have two Lithium Ebike batteries that aren't getting used over the winter. The Ebike batteries (1-48v, 1- 24v) could be easily unplugged and brought in to charge. I have a couple of DC/DC converter options although neither are perfect. I'd like some input on which of them might be better and is there something I'm missing that would make this a bad idea.

First choice would be a 30 amp GE converter from a forklift. It is a robust, but large, unit that will convert either battery to 12.37 volts output. That won't keep the battery fully charged but shouldn't allow it to go dead either. Dealer says it is not field adjustable but they can rebuild it. It's sealed but I don't know if it's potted or if I could open it up and change something inside.

Second option would be a small 8-10 amp max. dc/dc that would only work with the 24 volt pack. I don't think that is enough to keep up with the loads from the lights, but has the advantage of an adjustable output and I can set the voltage wherever I want.

I'm thinking I would need diodes on either one to block charge current from the alternator, do you guys agree?

[Stubby79]

You can get buck converters from ebay for cheap... for example,
https://www.ebay.ca/itm/200W-15A-DC-...0AAOSwr~lYrBBB for $10. There are plenty to choose from, and these ones can adjust the output voltage. You just need to wait for it to arrive from wherever it's coming from. Might be similar to your 24v one.

I doubt you need a diode, but it wont hurt to put one in. It will drop your output voltage a bit, and you may need to compensate.

Oh, and if all you are doing is EOC...unless you're coasting for 15 minutes at a time, I wouldn't worry about your existing starting battery to be able to keep up...unless you've gone and put a tiny lithium pack in or something. You should have plenty of reserve time in it to coast for a minute or two without a bother.

[jakobnev]

I built one of these. It's 14.8V 13Ah, and I connected it to the car through both filaments (in parallel) of an H4 headlight bulb.

I doesn't provide as much current as the alternator of course, but when you EOC >85% of the distance it certainly helps. Before it, I had problems keeping the battery charged in the winter, when I always needed the head lights. (Still dark when driving to work, and already dark when coming home)

[Stubby79]

You could also run a buck/boost converter from any 12-volt battery so that it outputs more than 12 volts to your system. A diode would allow it to recharge when the engine is running (though it will add load to your alternator/engine)

[puddleglum]

Originally Posted by Stubby79

You can get buck converters from ebay for cheap... for example,
https://www.ebay.ca/itm/200W-15A-DC-...0AAOSwr~lYrBBB for $10. There are plenty to choose from, and these ones can adjust the output voltage. You just need to wait for it to arrive from wherever it's coming from. Might be similar to your 24v one.

I doubt you need a diode, but it wont hurt to put one in. It will drop your output voltage a bit, and you may need to compensate.

Oh, and if all you are doing is EOC...unless you're coasting for 15 minutes at a time, I wouldn't worry about your existing starting battery to be able to keep up...unless you've gone and put a tiny lithium pack in or something. You should have plenty of reserve time in it to coast for a minute or two without a bother.

Yes my small converter is similar to that one, but a little different. I might pick one of those up though. Looks interesting and has a wider voltage range than mine.
I was concerned the reverse power output from the Alt. might damage the converter without a diode. You don't think it would?
I've never had problems yet with the battery only, but I have really short trips, mostly in the dark now with lights and it's pretty cold here. All that together is really hard on the battery and it never runs long enough to get fully charged by the alt.. That's why I think It needs some help. I've been charging at night but I'd like to ease the load on it.

[puddleglum]

Originally Posted by jakobnev

I built one of these. It's 14.8V 13Ah, and I connected it to the car through both filaments (in parallel) of an H4 headlight bulb.

Good idea. I thought about doing something like that as well, but the ebike packs are ready to go. Are you using the headlight bulb to cut back the voltage instead of a DC/DC converter? 16.8 volts charged is a little high.

[Stubby79]

Originally Posted by puddleglum

Yes my small converter is similar to that one, but a little different. I might pick one of those up though. Looks interesting and has a wider voltage range than mine.
I was concerned the reverse power output from the Alt. might damage the converter without a diode. You don't think it would?

That was just the first once that I came across that could handle most ebike voltages. If you're not running a BMS (because your pack isn't lead), just watch out that it doesn't drop your battery voltage past it's lower limit; these things don't have a set low-voltage shut-off.

I doubt that it will be bothered by there being voltage on the output side when it isn't being powered. You can/probably should use a multi-meter and see if any current is flowing back in to it with it powered off. Don't want to find your main battery dead because this thing has been trickling it off slowly. Or just go with a diode and have some peace of mind on both counts.

[puddleglum]

Hey, one other question. Can you see any problem with running two of those little converters in parallel? It seems like it would work to me. the only issue I see is one may cut out slightly before the other but I can't see that hurting anything.

[Stubby79]

Shouldn't be an issue. At worst, one works harder than the other. As long as it isn't working tooo hard.
Chances are, they'll both be working roughly equally hard, as long as they're set the same.

[teoman]

Get 2 and use both batteries. set the output voltage of one slightly lower so it only kicks in after the first has been depleted.

or put them both on a relay so you are either charging or discharging your batteries.