12 Volt Cranking Battery Charge to Caravan on The Road

thalls01 · Veteran Member

12 Volt Cranking Battery Charge to Caravan on The Road

dabbler · Guru

Alternator charges batteries via dcdc. When stationary I used to operate a big inverter with a idling engine but now use a lower amperage inverter permanently connected to aux batteries. I've used it to recharge cordless tool batteries when moving.

thalls01 · Veteran Member

Hi dabbler, that was useful 'food for thought'.

I had wondered how off-roaders using an inverter kept their AGM batteries charged and did not allow them to fall below 50% of their capacity (typically something like 9.7 volt) without constantly scrutinizing a volt gauge.

Having the engine idling and the alternator charging the AGM batteries to power the inverter would be a good means around this. Not a bonus I had considered - thanks.

In terms of solar charging vs alternator charging via an Anderson 100A cable, I assume the alternator would win out. Would I be correct? Just attempting to determine the benefits of outlaying on additional solar to my 150 watt fixed panel on the caravan.

In terms of keeping 2 x AGM batteries charged, would you outlay on the alternator method entailing purchase of a DC-DC device before additional solar?

For 2 x 120Ah AGM batteries a 1000 watt inverter would draw??

Peter_n_Margaret · Guru

Solar first because it operates wether the tug is running or not. It is a "set-and-forget" solution and continues to do its job when the van is parked up or not in use.
Charging direct from the alternator is also possible witout a DC-DC charger, depending on the size and length of the cable and the resulting voltage drop. If the voltage drop is low, as is our case, the charge rate can greatly exceed that achieved with DC-DC.
Cheers,
Peter

-- Edited by Peter_n_Margaret on Friday 18th of March 2022 10:49:35 AM

Jaahn · Guru

Hi smile

I see you are just asking for ideas to make an informed choice what to spend money on !! Good to do that first. smile

I have a small MH so it is a bit different but similar. As my bus is older I can just use the alternator direct with a VSR and the cable run is shorter. Newer cars have variable voltage alternators and with a long cable run also, you need a DC-DC charger to make it work well. Plenty of discussion here about that, and also in the Solar sub forum above. I have solar for normal battery charging and a small inverter.

If you drive every day then the alternator works well. But if you camp for some days then you need something for the batteries. Solar is good mostly but you would need more. Then use the alternator as a backup. However really you do not want to have the car engine idleing too much too often. A DC-DC needs to be bigger than a lot of people buy if you want quicker alternator charging.

I think you should consider what you want to run and the power used per day. Then how to put that much power back plus a good margin for bad days. Work it out in amp hours.

Jaahn

-- Edited by Jaahn on Friday 18th of March 2022 10:58:39 AM

Whenarewethere · Guru

We only have a car. The alternator is 180amps (new models 210amp).

Theoretically we can go 7 days to power the fridge off auxiliary batteries (four x 26AH also can be used for jump starting, up to 1040 amps)

We have two DC-DC Victron chargers, can be wired in parallel in unlimited numbers. As much as your alternator/s can cope with without cooking.

Since our maximum stay in one place is about 4 days we don't even plug the solar panels in.

So on paper, if we don't move we are ok for up to a week without sun. Then could run the car to charge batteries.

A well designed solar setup in series will also give you a few amps in crap weather.

All our wiring is totally over sized as not to waste any energy. Can even use the welding cable jumper leads to connect the solar panels. Overkill but it does give us another 3 metres if desperate to reach the sun.

We try to make everything as multi tasking as possible.

Whenarewethere · Guru

When calculating your wiring resistance. The resistance is the total metres of both + & - so if you have 10 metres from tug to trailer, it is actually 20 metres.

dabbler · Guru

I use my inverter to power AC stuff that either has no DC equivalent or the DC equivalent isn't worth the expense (my Dewalt battery charger for instance). I have once used an inverter to test my AC charger in recovery/repair mode just so I knew I could but have never needed it in anger A DCDC charger will never draw more power than a correctly functioning alternator can provide because they are electrically and electronically limited to a max amperage well below an alternator's output. I've had two (25 amp and 40 amp) fitted to my two battery banks for 4 years with no issues. And I no longer monitory battery state constantly even though ai have several ways, because I know my systems & know their limits and foibles.

thalls01 · Veteran Member

Thanks everyone for the input!
Good to have a diversity of views to ponder and consider.

I think I will try and incorporate both measures: BMPRO's DC-DC charger that has isolated distinct inputs for solar and cranking 12V charge to the AGM batteries + additional solar cells, potentially 180 to 200 watts.
BMPRO's unit that has both inputs can cater to 50amps from memory. You can set and forget settings for solar to charge the caravan's AGM batteries first, and the cranking battery 2nd.

At the same time when the engine is running, the unit via the alternator will charge the caravan's AGM batteries 2nd to the cranking battery.

With an additional portable 200 watts to my fixed 150 watt cell on the caravan's roof connected directly to the AGM batteries via a BC300 shunt to the BMPRO35 system, I should also be able to remain camped in the one position for longer.

dabbler · Guru

Any decent DCDC charger should incorporate a solar circuit too. The cost of cable, connections and fusing is minimal compared to the cost of most tow and trailed vehicles you're pretty much guaranteed to arrive in camp with fully charged batteries. If you have space and weight to spare, an extra portable solar panel is worthwhile especially is you like to camp in the shade.

thalls01 · Veteran Member

Hi dabber, thanks for that advice/summary. That is in accord with my thoughts also.
Cheers and thanks, Tim