Charging 12 volt battery from a larger 12volt battery

[AzRoute66]

Then perhaps the PST-BC1212-15 from that same page. Adjustable current limit and floats after absorb. No second PWM controller. If not, I'm sure the right one exists. Usually finding the first few can open up the flood gates to many similar models.

[sensij]

I don't think I understand the purpose of the second PWM controller. If I did, I would have addressed it along with the 12 volt chargers.

I think the idea is that PWM1 charges the 60 Ah battery, using PV as the source. PWM2 charges the 7 Ah batteries, using a boost circuit as the source, which is fed by the 60 Ah battery. I'm not sure what the PWM really offers here, except I guess some ability to reduce the boosted voltage back down to a float voltage for long term maintenance. It seems like a boost charger with adjustable output would eliminate the need for PWM2.

The PST-PB1108 only boosts to 13.8 V, and I'm not sure what the 7 Ah battery will want as the charge voltage.

[AzRoute66]

I don't think I understand the purpose of the second PWM controller. If I did, I would have addressed it along with the 12 volt chargers.

[Raypat]

Thanks AZ. These are the DC to DC chargers I mentioned in my original post but there is one in there (PST-PB1108) that is not all that expensive. $200 Australian delivered and it might do the job. It says it is constant current constant voltage so I am unsure if leaving the 7 amp hour batteries connected to this unit for days and sometimes a week or two would harm them or whether it has an inbuilt charging algorithm that would protect the battery being charged. Anyway thanks again for taking the trouble and I will ask the distributor if it will work for my application.

[Raypat]

Thanks for the response Mike but I don't think you are correct in your assumption that you can't charge a 12 volt battery from another larger 12v battery. That is the purpose of the buck/boost converter in the setup. It boosts the 12v up to whatever voltage you want within the limits of the device albeit at the cost of increased current draw. Which should not be a problem in this case given the size difference in the two batteries.The thing I am unsure about is hooking two charge controllers up in the manner suggested in my original post. I was hoping someone on here had done something similar. Charging at home is not an option as the glider is owned by a syndicate of three and we all need access to the batteries.

[AzRoute66]

This page might contain multiple chargers that wouldn't need an intermediate inverter/charger combo.

https://www.powerstream.com/DC-input-chargers.htm

[Mike90250]

you can't charge a 12v battery, from a 12v battery. Lead acid batteries need about 14v or so, to be charged from, you need the couple extra volts to force the charge into the battery.

the simple thing, is to pull the batteries out, and charge them at home

Next simple thing is to mount a " milk crate" to a hand truck, and put a 90ah deep cycle battery in it, wheel it back and forth, and use a small 120V inverter to power the charger for the 2, 7ah batteries overnight.

Adding about 200W of solar to the hand truck would be the next step up.

But you can't just hook 2 batteries together and have one charge the other. The "chargee" will never get a full charge from the donor battery, you need the inverter to power the proper charger for the glider batteries. The other gear, you run off the large 90ah deep cycle battery.

[Raypat]

I need to charge my glider batteries 2x7AH SLA from a larger 12 volt battery. Also would like to run say four 800 lumen lcd lights occasionally and from what I have read they should draw about 1 amp each and maybe a 12 volt soldering iron also occasionally which I think should draw about 4.5 amps. The 7 amp hour batteries would need to be charged overnight so that consecutive flying days would not cause any problems. The only option for power in the hangar is solar. I am thinking of getting a 100w panel connected to a 60 Amp Hour battery through a 20 amp PWM solar controller. I do not know much about solar or electronics in general but from what I have read I am thinking it should be possible to connect a buck/boost converter to the load terminal of the PWM controller to step the voltage up to whatever is required to charge the small batteries and then connect the output of the buck/boost to the input side of a second PWM charge controller to take care of regulating the charge so the small batteries are not overcharged. The setup downstream of the large house battery would only cost in the order of $100 or so and is the main reason for my thinking. I have seen commercial dc to dc chargers for sale which would do the job but they are several hundred dollars. Is this a case of you get what you pay for or is my system a workable solution? Any alternative suggestions or opinions on this are most welcome.Thanks in advance.