Dual Car battery charger

[discostuee]

Hi, is it possible to make a dual battery charger.
Both batteries are not linked and sick of climbing in the boat and under the seats to attach the charger, i would like to make 1 anderson plug at the back and just plug that in and it charges both?

thanks

 

[MikeMl]

I use a solar panel to charge two batteries in my boat. The boat is stored under a roof, so the panel is on the roof; not on the boat. The dual charger is external to the boat, so there are three wires going from the charger to the boat; B1+,B2+, and Common. I use a polarized triple assembly of Anderson Power Poles to make the breakaway connector.

The dual charger is smart enough so that if one battery develops a shorted cell, that would not prevent the other battery from charging as would be the case when using two diodes to isolate one battery from the other with a two wire system...

I suppose I could mount the charger inside the boat, and then there would be only two wires going from the solar panel to the boat when it is parked in the shelter...

 

[dr pepper]

There are commercial dual chargers such as the adverc and silverline systems, they are very expensive.
If your charger produces enough voltage, like 14.5v or more you can use a double diode, you can buy these from chandlers too and they are much cheaper.
Using a double diode with a 'dumb' type charger isnt so bad, the voltage drop across the diode doesnt affect the battery charge that much, allthough the charger will charge the lowest battery first, and if one is dead then both dont get charged.
Using a double diode with a alternator on the other hand isnt good as the voltage drop across the diode does have an effect, the batteries instead of being charged to 13.8v are charged to 13.8v - 0.6v (diode drop) = 13.2v which cam have an effect on bettery performance.

 

[discostuee]

Hmm, well the batteries are both deep cycle marine batteries and separate but did want to link them with a solenoid so it charges both when the motor is running, but saying that ive been out at night and had all things on and the aux battery has never gone flat so i might not link them.
So as the charger i have recognizes when the battery is charged and goes into trickle mode, how does it work if i have both hooked up with the diodes, if 1 was 95% charged and the other 30% charged wouldn't the 95% over charge while in the time the 30% is charging?

MikeMl, i was thinking of doing that with the solar panel but only good in the summer months (mine is under a carport too)

 

[MikeMl]

There are commercial dual chargers such as the adverc and silverline systems, they are very expensive.

I built my own.

...
Using a double diode with a alternator on the other hand isnt good as the voltage drop across the diode does have an effect, the batteries instead of being charged to 13.8v are charged to 13.8v - 0.6v (diode drop) = 13.2v which cam have an effect on bettery performance.

The right way to do this is to use a remote-sensed alternator controller that senses the primary battery voltage on the cathode-side of the isolation diode. That effectively puts the forward diode drop inside the feedback loop so it has no effect on the charging process.

 

[MikeMl]

My solar system floats the two batteries independently. I have the typical Off-Bat1-Bat2-Both battery switch. I just turn it to OFF while the boat is parked.

I have no source of "equalization charge" because the boat is stored miles from commercial power. I diddled the voltage regulator on the alternator to bump the battery voltage to ~15.5V on demand so that I could equalize one or the other battery while running down the lake.

I find that equalization is important for battery longevity when the boat sits on the trailer for up to 11 months a year without moving...

 

[dr pepper]

Disco stu, I assumed you were not that well up on electronics, but then you did imply you wanted to build a charging system.

You could build a ideal charging system with a switch mode converter, so both batteries get the correct charge, or you could build something a little simpler depending on your skill/time.
If your charging your batteries from the mains, then a very simple approach is 2 chargers.

Marine alternators usually have a third terminal for sensing battery voltage, as marine applications often have long cable lengths and isolation switches to introduce vdrop, however outboards and car engined boats often dont have this.

 

[discostuee]

Ok so ive been looking at some floating trickle charger schematics and think if i make 2 of them and put on each battery then hook them to a 12 anderson power source that would keep the batteries in perfect form. The boat sits there for around 1-2 weeks each time so they should charge in that time. I will just keep them not linked as ive had no need to over the past couple of years and dont want to add extra to my workload. Does this sound more suitable?
thanks

 

[discostuee]

this one looks good,
https://www.aaroncake.net/circuits/charger2.asp

or this one
**broken link removed**

whats your thoughts, they both have the float charge and i can make 2 and put 1 on each battery permanently installed, then hook them both to a 15v power supply anderson connector at the back of the boat.

 

[discostuee]

?

 

[analog1]

if the charger can handle the batteries charge currents then all you need 2 diodes in series You will loose .7 volts on both batteries but that will work. Or add a pull up resistor to a higher voltages to bias the diodes. but .7v is not loss is ok too.

 

[dr pepper]

I have to differ (if I understood the question right), 0.7v is a large part of a lead batteries charge, the cells wouldnt give their max energy storage and the batteries life would be shortened.
A 12v batt with 12.7v on it 20 mins or so after its been charged is considered full, 12.7 - 0.7 or 12v is considered discharged.

 

[joat]

how about simpler ... a NC /NO relay with a car flasher unit in series with the relay trip contacts ....

it flips the charge between the 2 batteries as the relay opens and closes ...

 

[stuee]

Im back onto this again i like your idea joat but i was thinking what if i have it setup so when battery one reaches 14.??v then it turns on the relay and the charge goes to the other battery. Just not sure how to do this. thanks