Hi there all,

I am after some assistance with an idea that I believe should be simple to achieve, but I do not have the technical expertise to design – I can follow circuits and build them, but cannot design.

I have a situation where I have two 12V batteries in a boat, but the boat is not used regularly. I want set up a battery charging system using a small solar panel (probably 5 to 10W) to maintain the batteries in a charged condition.

The batteries are isolated from the boat circuits using a rotary isolator switch that can be set to ‘OFF’, ‘BATT-1’, ‘BATT-2’, or ‘BATT 1+2’.

I intend to install separate wiring from the batteries direct to either a 3-pin charging plug/socket (Bat1+ve, Bat2+ve, Common-ve) or two separate 2-pin plugs/sockets to connect the charging circuit – this wiring would bypass the isolator switch.

To alternate which battery is charged, I hope to have a circuit that will perform as follows:
Day 1: Sun comes up, the solar panel generates power, the control circuit activates and directs the power to Battery 1. Sun goes down, charging ceases, circuit goes to ‘sleep’.
Day 2: Sun comes up, the solar panel generates power, the control circuit activates and switches over to direct the power to Battery 2. Sun goes down, charging ceases, circuit goes to ‘sleep’.
Day 3: Sun comes up, the solar panel generates power, the control circuit activates and switches over to direct the power to Battery 1. Sun goes down, charging ceases, circuit goes to ‘sleep’.
And so on…

Considerations:
1. The circuit needs to run from the solar-generated power, which will be in the region of 12 – 18V, so appropriate modifications are required to get this down to transistor or flip-flop voltages.
2. As the power will come from the solar panels, and there will be no actual ‘switch’ to turn the system on, protection will be required to make the circuit stable against flutter etc (the solar panel output will gradually rise as the sun comes up)
3. Although I do not expect the battery charging current to be high (less than 1 Amp) I expect the circuit will need to operate a relay, although I gather high-power transistors could be included in the output circuit to handle the current involved.
4. In general, I am not fussed if the ‘system’ goes to sleep and then re-activates due to shade on the solar panel etc during any particular day. Overall, both batteries should be charged reasonably evenly.

As I mentioned earlier, I do not know how to design such a circuit, but I have a ‘general understanding’ of electronics. It seems to me that a flip-flop may be used, and most likely a relay in the output. But I do not know if the contacts of the relay should be used to change from Battery 1 to Battery 2, or two relays driven by the outputs from the flip-flop (assuming a flip-flop is the best system) - or for that matter if a flip-flop (or what type) would be the best approach.

I have done some ‘Googling’, but not found something that looks like it will answer my situation, so I would appreciate any advice from the forum.

Many thanks in advance,
Ian

 

Ok well I'm personally in your boat (figuratively). I can't do anything with the circuit design but I can make one if I see it. I can, however, give you just a little personal opinion on the solar aspect of this. I have done extensive research on solar cells in relation to charging batteries. I've found that its better to look at solar panel amps, not watts when charging batteries. They only need about 2 amps to do a trickle and complete charge at about ±12.8 to ±14 VDC. Any more than that and you could run the risk of boiling the lead-acid and exploding the battery (very nasty to clean up). If you wire the batteries parallel to the panels, they should charge evenly. I will warn you right now though. This is only from what I've taught myself. I'm just a very opinionated high school sophomore that knows a bit about this stuff, so, naturally, I voice my opinion. I hope i could help in some way!

Mishael

 

Thanks Mishael.

I am happy that no larger than 10W will do what I want, but definitely want to charge the two batteries separately. It is not a good idea to have them running in parallel unless they are known to be a balanced pair - better to avoid the situation.

If I can get a circuit to switch automatically each day when the panels power up the circuit, I'll be happy.

cheers,

Ian

 

I would take the panel output and use one diode to each battery. The one with lower voltage will take the most current. No need to switch.

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