Coupling two 12V PV systems together - the best way? - Page 1
Forum section: electronics / general

author: DTJ
author_rank: Super Contributor
timestamp: December 28, 2020, 03:28:40 am
content: I've got two small 12V PV systems, one on my vehicle and one on my camper trailer.If my camper trailer is parked in the shade I'd like to be able to charge the battery from the solar panel on my vehicle via a cable run between the two systems.If I run a cable directly between the two batteries I could end up with a situation where the fully charged batteries in the vehicle try to rapidly charge the camper trailer battery.Another way would be to put a 5A or whatever active current limit in the cable - doable but it means some work.Alternatively I could place a changeover switch in the vehicle that isolates the vehicle batteries from the charging system and connects it to the cable that runs to the camper trailer battery.Hey I could even use a relay to isolate the vehicle batteries when the camper trailer cable is plugged in.I'm not sure if having a 10m cable from the vehicle MPPT controller to the battery will cause any issues (other than resistive losses of course).Anyone got some ideas on the best or simplest way?For all all of the stuffing about maybe its easiest to just grab a standalone panel or blanket.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 28, 2020, 02:20:11 pm
content: The charging and discharging voltage ranges for lead acid are far enough apart that it would have to be quite an imbalance for a large current to flow. A circuit breaker will be enough protection.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 28, 2020, 04:26:34 pm
content: The attached could be ok if charging current wasnt too high.Low voltage schottkys can have very low Vf.Otherwise i think you would need some circuitry involving a Hall effect current sensor eg by allegromicro.Perhaps a little micro or arduino in there to switch on/off back-to-back fets which will have much lower volt drop than the schottkys.The logic would be.....switch the fets on...if theres a big current flow, then switch them staright back off again...the battery could handle a high current surge as long as it wasnt for too long. But you'd need fets rated big enough so they could break that high current, and youd need snubbing due to circuit stray inductances.I guess youd have to use the battery voltage as your bias power for the circuitry and have the fet switch right by the battery.

author: DTJ
author_rank: Super Contributor
timestamp: December 29, 2020, 06:27:56 am
content: Thanks guys. I'm going to think about this for a bit.

author: johnwa
author_rank: Frequent Contributor
timestamp: December 29, 2020, 06:48:28 am
content: Quote from: NiHaoMike on December 28, 2020, 02:20:11 pmYes, the wiring resistance will probably limit the current enough. If you are still concerned, add a headlamp globe in series. And a Schottky diode if you want to stop reverse current flow.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 29, 2020, 01:38:27 pm
content: What about a mechanical switch , but it goes into a small simple electronics module...then the module measures the voltage of each battery...and decides whether a relay actually gets switched on or not.......ie connecting the "sunny" panel to the "shaded" battery aswell as the "sunny" battery. If the batteries are the same voltage, then the relay will switch on...if not, then it wont.Alternatively, connect each battery together with chargers, one going one way, and the other going the other way.....obviously only one charger would work at a time (but you could have them working together of course, just wouldnt be too much point). They could even use much of the same wiring to the batteries.