Dual Charging Opportunity with Solar and Vehicle Battery

[whfarms]

If I have a main battery that is connected to solar through a Victron charge controller and another 12 volt battery that is in a vehicle and I want to connect the two batteries so that if the vehicle is running I am supplying 12 volts (basically taping into the ignition key so only charges when the vehicle is on) do I hook the battery in the vehicle and the solar panel both to the single charge controller that controls the main battery.

I may need a DC-DC charger instead of hooking the lead acid battery directly to the charge controller.

Thanks in advance.

 

[Dave in AZ]

If I have a main battery that is connected to solar through a Victron charge controller and another 12 volt battery that is in a vehicle and I want to connect the two batteries so that if the vehicle is running I am supplying 12 volts (basically taping into the ignition key so only charges when the vehicle is on) do I hook the battery in the vehicle and the solar panel both to the single charge controller that controls the main battery.

I may need a DC-DC charger instead of hooking the lead acid battery directly to the charge controller.

Thanks in advance.

You need a dc to dc charger.
Terminology-- it helps if you use standard battery terminology. So, not main and battery in my vehicle. There are starter battery, the normal flooded lead acid one all cars have. And there is a House battery, which is usually a LiFePO4 lithium battery these days, to run camper loads like lights, fans, diesel heater, etc. There are also 2nd starter batteries, just to increase starting power.

Alternators are smart now and vary the voltage as loads change. Old isolator setup doesn't work sufficiently now.

Every company makes 4 or 5. There are 1000 threads on charging A 2nd or house battery from your alternator using a dc dc charger.

The dc to dc charger isolates the two batteries so your start battery doesnt get drained. Protects alternator from high fast loads possible with lithium. Provide correct voltages in 4 stages to properly charge lithium. Limit amps from your alternator so you don't burn it out overcharging. All that is needed.
Alternator should be run at 50 to 60% max capacity, not more when extended, or you will overtemp it and ruin it. Without a large alternator, 25A to 30A is probably most you should pull. They have 50A ones, be sure your alternator can handle that. Know your car's electric load with all stuff running, and alternator size.

Victron makes good ones, the Orion line.
Redarc makes good one, called BCDC25, BCDC40.
Kisae makes a good one.
Renogy makes one that also has SCC controller, but they break a lot, generally get replaced later with victron.

 

[scrubjaysnest]

What kind of batteries? What I had was solid state battery isolator from an RV dealer. Worked great with lead acid batteries for years. As mentioned above with lithium a dc to dc is needed. I leave the solar on all the time

 

[whfarms]

Well my started battery is a lead acid battery on my tractor that is about 70 years old and my house battery is a LiFePro4 battery that is on a wagon that waters cows. It drives a 12 volt diaphragm pump that is wired to a pressure switch like you would use on a well. When the cows drink from the tank the pressure in the hose drops and the diaphragm pump kicks on.

The house battery normally charges with 4 200 watt solar panels through a Victron MPPT 100|50 charge controller. Works great 99% of the time but a week of clouds can get the best of it.

The alternator isn't smart. Its probably 30 years old. I assume is a Delco. Works just fine.

End goal is to give it some juice from the tractor when the tractor is running. It has to come in twice a day to be filled so it would run for at least 10*4=40 minutes no matter what and if I needed to let the tractor run for an hour it isn't a big deal uses almost no fuel. In addition it takes about 45 minutes to fill it so I could also have a 120-12volt charger on it for those 45x2=90 minutes a day.

So between all three I hope to keep it charged up when the sun isn't at its brightest.

I will order a DC-DC charger. I need to do some research as it appears they are offered in a dozen different amperages.

 

[HarryN]

In WI, for that application, it probably makes the most sense to use a lead acid battery.

One approach is to use a DC - DC charger. Another is to put an inverter on the tractor that is powered by the tractor battery and plug the existing 120 vac charger into it. Turn off the inverter when not in use so that it does not drain the tractor's battery from parasitic losses.

The cost to implement will be roughly the same, so it is more about what is convenient for you in general.

 

[Dave in AZ]

@whfarms thx for the full answer. Cool setup. Ok, you can do exactly what you're asking, for pretty cheap. It really depends how big your house battery is, and how much charge you need to do daily.

1. Cheapest and easiest and fastest is to just plug into AC while filling, you can usually charge a lifepo4 battery at 50% of its rating, .5c they call that. So if you had a 100Ah 12v battery, you could charge it at 50A pretty easy. Most 100Ah batteries I see will take 100A in and out from their Battery Management System, so you could do more probably.
Just get a charger, plug it in. In 90 minutes, you could do 75 to 100Ah on a 100Ah battery, a full charge. A victron Skylla IP65 waterproof 70A charger is $765 though. .

2. if you still wanted the charge more off tractor, You don't need anything but an isolator relay on the alternator or start battery, and some cables running to the house battery. You could still run a dcdc if it's easier, just the old school pre-smart-alternator way works too.
Here is what I'd use, the original isolator guys, Redarc, and you can get the 100A one for $140.

Dual Battery System Isolators : REDARC SBI

Want to add a dual battery isolator to your 12v setup? Check out REDARC's dual battery isolator range today and see what it can do for your vehicle.

www.redarcelectronics.com

But you'd get like 50Ah probably in the 4 trips.

 

[Vigo]

An old tractor uses almost NO amps to stay running so you could potentially charge pretty aggressively off your tractor alternator if you used a DC-DC charger. I bought a 60a (output) model for $71 on sale from Amazon so it is POSSIBLE to get them cheap although you'd probably want to house it on the trailer so you could put a nice-ish enclosure over or around it. Not many good surfaces on a 70y/o tractor to put something like that out of the rain/weather without building it a 'box' somehow.

If you don't need a very high charge rate you could simply directly parallel the two systems (just hook positive to positive, negative to negative through something like an Andersen connector) and the alternator would charge the lifepo4 if it was low, and do 'not much' to it if it wasn't low. That's just because it takes a voltage above 14v to maintain a high charge rate into a lifepo4 that is not near empty and most alternators would rarely go much over 14 if they even get out of the 13s at all. However, when a lifepo4 is actually at a low charge it will fall into the 12s and lower, in which case you will still get a pretty aggressive charge rate from your stock tractor charging system. Direct alternator charging will get you a good fast charge rate from like 0-20% on the lifepo4, at which point it will slow dramatically but will still EVENTUALLY get the lithium close to full. This is fine if all you're trying to do is 'keep it from being empty'.

So if you are only worried about supplementing a 'low' battery, you can just directly hook them together. If you want your tractor to 'fully' charge the lifepo4, or maintain a high charge rate when the lifepo4 is already at a high state of charge, you would need a DC-DC charger to step that alternator voltage up to ~14.6 before feeding it to the lifepo4. They also do 'multistage charging' and have low voltage cutout, etc etc.

 

[eXodus]

That's just because it takes a voltage above 14v to maintain a high charge rate into a lifepo4 that is not near empty and most alternators would rarely go much over 14 if they even get out of the 13s at all.

this is what I experienced on two different vehicles. At 13.8V (what many alternators are running) I am getting 30-50A, maybe 60A when the LFP is very low, from either a 110A or a 180A Alternator.

The LFP charging from the alternator is slow! Those stories off burning out alternators are kind of overstated. I have found ONE person on this board which burned up an alternator, which was on boat (very different usecase) - most everybody else is complaining about it being slow.

charging directly without a DC-DC is no ideal for LFP. But, very possible - just monitor you amps, and see what the alternator is doing.