Moving to 24v system, requires step down for house DC?

[phemens]

I am considering replacing my existing Samlex EVO-3012 inverter charger with a 24v unit (to repurpose the Samlex for another application.
This would work well with my battery bank, I could rewire the 6 batteries into a 3x2 24v array. I'm looking at the Victron Multiplus 3000/24, since all my other equipment is Victron and the integration would be nice.
So if I do this, I guess I would need a 12v step-down converter to supply house 12v DC? Any recommendations? Would need to supply for the usual (lights, pumop, furnace, LP detector, radio) plus landing gear, macerator pump and water transfer pump.

[3_tons]

valhalla360 wrote:
So what is the goal of setting up a 24v system...if you aren't using 24v for all the DC loads?

24v systems are becoming more common on large yachts but that is because they have high wattage devices that are a long way from the battery bank and the voltage drop due to high amperage becomes a problem (Ex: Anchor winch might be 40-50ft of cable from battery to winch and a small winch might draw 250amp. By increasing the voltage to 24v, that drops to 125amp. They also may have electric sail winches, bow thrusters and other high wattage devices drawing hundreds of amps at 12v

By comparison, a slide out motor might draw 30amps and that's one of the bigger DC loads on an RV.

You’ve raised an interesting point here, one that I’ve often wondered myself?
Though I do not have long wire runs in my truck camper...I do occasionally run my 11kbtu air cond from the battery, but the biggest concern here is harmful LRA momentary voltage sag which can be reasonably addressed with a soft-start and with the rather stable voltage profile of LiFePo4...

FWIW, Victron BMV measured (at 12v) a LRA instantaneous pulse of 96a and steady running amps (after 12 min) of 88a...Using 0004, 6’ round trip cabling...

3 tons

[valhalla360]

So what is the goal of setting up a 24v system...if you aren't using 24v for all the DC loads?

24v systems are becoming more common on large yachts but that is because they have high wattage devices that are a long way from the battery bank and the voltage drop due to high amperage becomes a problem (Ex: Anchor winch might be 40-50ft of cable from battery to winch and a small winch might draw 250amp. By increasing the voltage to 24v, that drops to 125amp. They also may have electric sail winches, bow thrusters and other high wattage devices drawing hundreds of amps at 12v

By comparison, a slide out motor might draw 30amps and that's one of the bigger DC loads on an RV.

[MEXICOWANDERER]

Let me ask a simple way...

What is the fuse size for your jacks?

[phemens]

time2roll wrote:

phemens wrote:
So if I do this, I guess I would need a 12v step-down converter to supply house 12v DC? Any recommendations?

Victron has several to choose from. Only issue is if you will have a 12v battery buffer the converter needs to be rated for such. Any Victron should work as a direct power supply.

Thanks, not sure what you mean by 12v battery buffer? When you say any Victron should work as a direct power supply, do you mean it can manage the step down directly or as a source to a step down converter?

[phemens]

BFL13 wrote:
Not an issue with your batts (Li ?) If you have 60 cell panels instead of 72 there is a problem getting the voltage above 30v for 24v batteries. I ran an experiment with that and it stalled out just below 31v. I had the controller doing 24-24 on Flooded batts.

The pos wire from the breakaway switch to the 12v battery in the trailer is not supposed to have a fuse. Might need a way around that if using a buck converter for the 12v, not clear on the wiring for doing that.

You could make one pair of the 6 batts do 12v, and put the other 4 in series-parallel for 24v on the inverter. You need a separate charger for the 12v battery bank.

2oldman ran a 24v system for a while but changed up to a 48. He used a DC-DC converter for 12v things. Why go 24 at all when you can go 48 would be the question I suppose. Don't know how he ran his breakaway.

Thanks, the panels are 72 cell and in series, so no issue on voltage. Dedicating a pair on the batteries to 12v would seriously impair my inverter usage, I'm running a residential fridge off the batteries full time and they draw down ~ 6A/hr on it's own. Going to 48v would make sense if I added another pair, but I'm probably in my sweet spot for capacity right now.

[time2roll]

phemens wrote:
So if I do this, I guess I would need a 12v step-down converter to supply house 12v DC? Any recommendations?

Victron has several to choose from. Only issue is if you will have a 12v battery buffer the converter needs to be rated for such. Any Victron should work as a direct power supply.

[BFL13]

Not an issue with your batts (Li ?) If you have 60 cell panels instead of 72 there is a problem getting the voltage above 30v for 24v batteries. I ran an experiment with that and it stalled out just below 31v. I had the controller doing 24-24 on Flooded batts.

The pos wire from the breakaway switch to the 12v battery in the trailer is not supposed to have a fuse. Might need a way around that if using a buck converter for the 12v, not clear on the wiring for doing that.

You could make one pair of the 6 batts do 12v, and put the other 4 in series-parallel for 24v on the inverter. You need a separate charger for the 12v battery bank.

2oldman ran a 24v system for a while but changed up to a 48. He used a DC-DC converter for 12v things. Why go 24 at all when you can go 48 would be the question I suppose. Don't know how he ran his breakaway.

[DrewE]

For the breakaway brake system, I'd probably just connect it to one half of one of the battery pairs for general use in as much as the only time it should operate is when there's an emergency, and under those circumstances an uneven load on the batteries is the least of one's concerns.

The simplest and probably best overall method of getting 12V from your 24V battery bank is with the use of a "battery equalizer". With this, you connect your intermiedate 12V leads on all the batteries to the equalizer, and take the 12V from this lead. The equalizer has circuitry that basically forces the two 12V sides to have an equal votage at all times by "moving" current/charge from one to the other. (Technically, I think it's closer to a voltage-controlled buck/boost converter where the output, at the center voltage point, is always one-half the total input voltage.) There are a few nice things about such equalizers: first, the equalizer only needs to supply half the 12V current, the other half coming from the "lower" half of the battery bank directly. Second, instantaneousl current demands in excess of the equalizer's capacity can be readily accommodated, in as much as it will work to pull the batteries back into equal charge after one is depleted more than the other. Third, it's generally possible to use a 12V charger with them to charge the whole 24V battery bank.

[2oldman]

I have a Victron unit. Excellent quality. I step down from 48 to 12 at 30a. Works fine for slides and jacks.

When I had 6- 12v batteries I tried cheating and wiring up just one for the coach, but that didn't turn out so well. Throws it off balance with the others.

[phemens]

That's the question. I know there are several (many) users out there with 24v systems, wondering what their solution is other than maintaining a separate battery system for house 12v , which seems to defeat the purpose a bit...

[MEXICOWANDERER]

Will 40 amps of 12 volts work the slide motor?

[phemens]

Hadn't considered the brake system, good point, although nominally I would wire into the same stepdown. Don't have a charge line from the vehicle, have ample solar.
Primary concern would be the 'momentary' high discharge from the landing gear and slides

[GordonThree]

Depending on your 12v needs, you could use either one big step down, or point of load smaller step-downs.

Good quality LED lights should work fine with a 24v system, having built in regulation.

Something else to consider is your emergency brake system, and charge line from the tow vehicle. What's the plan there, separate dedicated battery?