Trailer TO Truck charging..

cmorrow

New member
Hey all, this was very hard for me to search since most results I found were about charging trailers via the tow vehicle, but I have a question about the opposite direction:

I have a group 31 agm in my trailer fed with a 100w Renogy panel and Zamp controller. I have a group 49 agm in my 100 series Cruiser.

If I am in a situation where I need to jump the Cruiser, I’d just use the trailer battery. Would it also be fine to connect the batteries with jumper cables while the solar panel is connected to the trailer, thus charging both of my batteries together? This could be useful if I were running a fridge in my vehicle without a dual battery setup in the Cruiser and parked for a longer period.
 

cmorrow

New member
Broadly speaking, Electricity flows from where its produced to where its consumed. It does not care which direction its going.
Great, thanks! It all seemed to make sense to me but I wanted to make sure there wasn’t some ‘gotcha’ I wasn’t considering.
 

john61ct

Adventurer
Yes wiring requirements are all the same, it's all just amps and volts.

But you will need more panelage to handle charging two batts if they're both depleted.

400W per 200Ah is a good ratio.

In sunny desert near the equator 200 : 200 may be enough.

Of course your Ah per day usage really is the main factor
 

dreadlocks

Well-known member
What are you running in the tow vehicle with engine off thats going to require it to have a jump start? If you have some persistent loads going on in the tow vehicle then perhaps consider a second battery setup for that.. or wire up those loads to run directly off trailer battery and disconnect the truck battery from those loads.
 

cmorrow

New member
What are you running in the tow vehicle with engine off thats going to require it to have a jump start? If you have some persistent loads going on in the tow vehicle then perhaps consider a second battery setup for that.. or wire up those loads to run directly off trailer battery and disconnect the truck battery from those loads.
As far as I know, hopefully nothing - assuming my fridge works correctly. I don’t expect to be camping for long periods without running my vehicle, I just wanted to make sure that I could if needed. I’m not opposed to a second battery, but I really don’t think I will utilize it much and am going to see how this setup performs as-is.
 

dreadlocks

Well-known member
sounds like you're overthinking it, if you hook your batteries together that's when you risk draining it say overnight when sun's not charging.. then you wont have a battery to jump the other with.

leave em isolated, if it really comes to it and you play radio too long in tow vehicle or something just use your jumper cables.. let your trailer battery soak up all the solar it can without sharing, its gonna need it.

the batteries are gonna ping pong when you hook em together, say your truck battery fully charged and your trailer battery close to discharged.. with solar hooked up your trailer will be charging from both solar and your tow vehicle until your tow vehicle's voltage equals that of your trailer.. then your solar will have to try to bring both back up with its minimal output.
 

cmorrow

New member
sounds like you're overthinking it, if you hook your batteries together that's when you risk draining it say overnight when sun's not charging.. then you wont have a battery to jump the other with.

leave em isolated, if it really comes to it and you play radio too long in tow vehicle or something just use your jumper cables.. let your trailer battery soak up all the solar it can without sharing, its gonna need it.

the batteries are gonna ping pong when you hook em together, say your truck battery fully charged and your trailer battery close to discharged.. with solar hooked up your trailer will be charging from both solar and your tow vehicle until your tow vehicle's voltage equals that of your trailer.. then your solar will have to try to bring both back up with its minimal output.
Yeah, that makes sense too, it’s entirely likely I am overthinking, haha. Thanks man.
 

mlrtime99

New member
Or replace that Zamp charge controller with a CTEK D250SA which is also a MPPT controller - it uses either solar or tow vehicle battery (or can stack them) to charge the house battery, or if on solar only will send voltage to the tow vehicle once the house battery is charged. Truly set it and forget it.
 

Photobug

Active member
Maybe carry a jump starter lithium battery. I have had one for a long time. It fits under my driver seat, has enough connectors to charge anything you can think of at 12V and can jump start my diesel engine. It can be charged at home at 120v or in the vehicle using a 12V plug.
 

rkfoote

New member
Should do without saying, but make sure it's fused appropriately. If you connect the starter and trailer batteries together and attempt to start the vehicle you're going to be trying to pull half the current (ish) from the second battery. If you're doing this over a piece of narrow gauge wire you could very quickly have a molten piece of wire between the two (same could occur if one battery is well charged, and the over depleted when you connect them).
 

dreadlocks

Well-known member
No it won't, trailer battery resistance is far too high when there's a battery a foot away from starter with much lower resistance.

Also fuses don't control current, just stop all current if exceeds limit.. if what you said was true, everyone who started their vehicle w/trailer hooked up would have blown fuses as a result.

You can connect a fully charged battery with a fully depleted battery using a 30awg cable with no fuse and nothing bad will happen, if the current exceed the wire capability the voltage would have long dropped below that of the depleted battery and it would not even charge.
 
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shade

Well-known member
Maybe carry a jump starter lithium battery. I have had one for a long time. It fits under my driver seat, has enough connectors to charge anything you can think of at 12V and can jump start my diesel engine. It can be charged at home at 120v or in the vehicle using a 12V plug.
Regardless of any other electrical wonderment, that's my advice. I have a Clore commercial grade jump pack that could be used in place of my truck's starting battery, if needed.

If solar fails, the fridge low voltage cutoff doesn't, the generator dies, or some idiot has to SCRAM the reactor after a high speed run to catch Ramius, the jump pack will always be ready to serve.
 

jeffjeeptj

Adventurer
I used Anderson SB-350 connector between the TV and the Trailer. In the TV is a Blue Sea
ACR and an Odyssey PC-2150. In/On the trailer is 200W solar, Blue Sky MPPT controller, and an Odyssey PC-2250. When plugged together the ACR would connect or dis-connect as voltage dictated. If I started the TV and drove off the ACR would pull in. As the sun came up when parked, the ACR would pull in. Both batts charging.
 
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