Charging 12v Starter battery from 48v house bank?

[Daley]

Hi there,

I have a school bus conversion in process that has a 48v lithium battery bank being charged via an mpp all in one and solar array.

I would like to keep the bus 12v battery charged up as I don't move the bus too often. It has 2x12v lead acid starter batteries.

Is there a fairly simple way of charging the starter battery from the 48v bank? I see victron have a 48v-12v dc to dc device that comes in different amperages. Would that be the easiest way forward?

Thanks for any info

 

[snoobler]

Yes.

 

[JoeHam]

Not sure what the Victron cost is but for such a simple device I would consider the cheap Amazon stuff:

https://www.amazon.com/SMAKN®-Synchronous-Voltage-Converter-Waterproof/dp/B00VR5TSUU/ref=sr_1_4?dchild=1&keywords=buck+converter+48v+to+13.8&qid=1602632354&sr=8-4

 

[snoobler]

I almost went there too. A few issues with a 13.8V converter:

1. If the FLA batteries are fully charged to start with, floating them at 13.8V will require frequent checking/refill of the electrolyte.
2. If the FLA are at a lower state of charge, they'll never be fully charged floating at 13.8V. They'll likely get to a level that they can minimize internal damage, but you will experience sulfation and capacity loss. (you really want to get a FLA to 100% SoC routinely).
3. 10A would suffice for floating, but for actual charging, you'd want something closer to this 30A unit.

If you're willing to mostly charge the battery with the alternator, float with the converter and top off the electrolyte every 2-4 weeks, a converter might be fine.

 

[Daley]

How does the victron differ?

I was looking at this one

Victron Energy ORI481224110 Orion-TR 48/12-20 DC DC Converter

Will that output always be 20amp? I was thinking of having a volt meter attached to starter battery and manually turning the dc to dc on if and when voltage on starter battery started getting low. Would that be advisable or is floating better? My batteries are older and unknown as they came with vehicle but they get the job done, I just dont drive it often and thought topping them up, essentially via solar, would be a good idea.

 

[snoobler]

Hmmm....

It's output voltage is adjustable, but it doesn't act as a 3 phase 12V charger. A more favorable float is 13.2V. I really thought those were legit chargers and not just converters.

So, with the Victron you could change the output voltage to actually charge the 12V, but you would have to adjust it down to 13.2V to float it.

20A is just the max it can provide.

Floating is better.

Of the two, I'd go for the Amazon one and keep an eye on the battery electrolyte every couple weeks until I get an idea of how frequent it needs to be topped off.

Another option... since you already have an inverter, simply get a 12V battery charger with a float mode on it and power it with 1w0VAC. Yes, you'll lose efficiency, but it will actually CHARGE the battery and is a pretty cheap solution. Once the battery is in float, it will only consume 5-10W.

 

[cinergi]

Hmmm....

It's output voltage is adjustable, but it doesn't act as a 3 phase 12V charger. A more favorable float is 13.2V. I really thought those were legit chargers and not just converters.

The 24/12 has a 3-stage charge profile in it. For some reason the 48/12 doesn't. I'm running into this design decision myself at the moment. I think I'll float a LiFePO4 battery at ~13.4 with it and run my RV 12V off of that.

 

[tazmann]

If youre running an DC to AC inverter you could just plug in a small trickle charger for the 12 volt system

 

[cinergi]

Yeah I am seriously considering have a 12vdc converter (trickle won't be enough - I have real 12v loads) .. the dc-dc converter is only 87% efficient so a 120vac-12vdc converter attached to my inverter would be reasonable. I think I'm gonna try for the dc-dc converter tho. The bonus is that it's isolated which is great because I want to float my 48v bank.