Dual battery charger - new idea

[stuee]

ok so i kinda worked out with 2x 2core cable going to a DPDT switch then to the car battery charger, when the light turns green flick the switch to do the other battery.

I was wondering if there was something i can put inline to relays instead of the switch where i hook to the the green led so when it turns goes it it flicks some sort of trigger to change to the other relay? 12v car relays.

any ideas.
Thanks

 

[AnalogKid]

Without a wiring diagram or schematic, no.

ak

 

[spec]

Hi stuee,

In priciple we should be able to do what you want, but as AnalogKid implies, we need more information. Could you give replies to these questions:

(1) what type of batteries? I sssume standard 12V auto.

(2) How much current does your batter charger supply: 4A, 10A 20A, 50A?

(3) Are you prepared to get inside the battery charger and do some minor modifications?

(4) If the answer to (3) is no would you be happy if the green LED were obscured

(5) What do you want to hapen when the second battery is charged and the green light comes on again?

(6) A circuit diagram of the battery charger would be ideal, but I doubt that you would have that.

(7) Could you upload a picture of the battery charger or a web link showing it.

 

[stuee]

Hi, ill try answer some questions
(1) what type of batteries? I sssume standard 12V auto.
12V Marine batteries, they are under the seats and a pain to keep taking the charger in and out and changin connectors over, so i ran the cable to the shed and looped under the carport when not plugged in.

(2) How much current does your batter charger supply: 4A, 10A 20A, 50A?
4200ma

(3) Are you prepared to get inside the battery charger and do some minor modifications?
Yes i dont mind tinkering.

(4) If the answer to (3) is no would you be happy if the green LED were obscured

(5) What do you want to happen when the second battery is charged and the green light comes on again?
I would like it to go back the the first so its like a trickle top up charge till i get to it and turn it off, or alternatively if its not too hard just do nothing after the 2nd charge is complete

(6) A circuit diagram of the battery charger would be ideal, but I doubt that you would have that.
No diagram sorry.

(7) Could you upload a picture of the battery charger or a web link showing it.
Cant find a link but will take a photo tonight.

 

[spec]

Hi stuee,

Sorry about the interrogation, but the data is needed to get you a good design.

What you have said helps a lot. So my answer to your initial question is, yes, it can be done and shouldn't cost a lot or be too complex or bulky.

I have a circuit in mind but need to work out a few details. As I see it, a good way to impliment the function is to do the switching with a standard 1 pole change-over, 10A, 12V auto relay. The control should be fairly straignt forward using an LM555 timer chip.

Re (5) above, how would it be if once battery 1 were fully charged, the battery charger switched over to battery 2 after say 5 minutes, and then charged battery 2. 5 minutes after battery 2 is charged, howevever long that took, and the charger LED illuminates again, if the charger were to switch back to battery 1 again. If battery 1 were still fully charged 5 minutes later the charger would switch back to battery 2 and so on. If battery 1 were discharged, it would charge that, once again, no matter how long it took. I only ask because it may make the design simpler. It does have the advantage that you could leave it running and both batteries would be kept topped up, but this depends on the type of charger you have. Most auto batteries shouldn't be too bothered about 4.2A (4,200mA). You could specify any time you like; I just used 5 minutes as a suitable example. I think this operation would suit you from what you say.

In terms of the physical implementation there seem to be two broad approaches.

(Approach 1) Don't modify the charger at all; just tape an opto sensor to the charger front panel so it sees the battery charger LED and have the rest of the electronics in a separate waterproof box.

(Approach 2) This would be the neatest and most user friendly, but we would need to know the circuit of the LED. I suspect this could be fairly easily derived by simply looking inside the battery charger.

If there is room it would be best to have the controller inside the battery charger with two wires exiting for the two batteries. This would seem to be the best approach especially for a marine environment, and in view of the fact that you dont mind 'tinkering'.

Finally whichever approach you decide on I would recommend a 10A fuse in each of the battry cords unless the battery charger already has over current protection.

Three more requests for information:

(2.1) If you have a multimeter could you measure the output voltage of the battery charger with no batteries connected? Two readings are requested: with the multimeter set to DC volts (probably around 20V reading) and another with the multimeter set to AC volts (probably about 14V reading). If the AC reading is around zero, I would be particularly interested.

(2.2) Could you let us know the rating of the batteries, normally given as A/H? Failing that, if you could confirm that the batteries are lead/acid like normal auto batteries and also the dimensions of the batteries. That will alow us to make an educated guess as to the capacity. Better still, could you post a link to a similar marine battery- I'm a land lubber!

(2.3) Could you post the make and model number of the battery charger?

PS: can we know where you are from?

 

[spec]