How do I charge two batteries on one charger

[SmellyCat]

I have a regular car starter battery and a marine battery jumper cabled together. Right now I hook up a little Schmuncher??? 1.5 amp charger. Over night. I have no idea if I'm overcharge ring or undercharging the 2 batteries. what do you think I should do?

[shovel]

They're in parallel? You're just fine. It will charge them both at a rate of 1.5A to appx. the same maximum charge level as it would charge either independently.

[ksa8907]

keep all the positives together and all the negatives together and you're safe.

[SmellyCat]

Coolio, so I have deleted the Alternator and They Seem to be getting me to work an back. Am I thinking right that the starter battery will release its energy fast when I start the car, thus keeping the marine battery charged to run car longer?

suppose the starter battery is at full charge and the marine battery needs charging. will I be overheating the starter battery if it is charged and linked the marine battery when the charger is on? Sc

[shovel]

Originally Posted by SmellyCat

Coolio, so I have deleted the Alternator and They Seem to be getting me to work an back. Am I thinking right that the starter battery will release its energy fast when I start the car, thus keeping the marine battery charged to run car longer?

If they're both in parallel with no isolators or anything? No, they'll both share the work of turning the starter and of running the fuel pump etc.

Originally Posted by SmellyCat

suppose the starter battery is at full charge and the marine battery needs charging. will I be overheating the starter battery if it is charged and linked the marine battery when the charger is on? Sc

On 1.5 amps? No. Both batteries will necessarily lose and gain voltage at the same rate.

[SmellyCat]

Ok. I guess. It would be best to run the starter on the car battery and the car on the marine battery separately. it might not be hard to connect the car battery to the big hot wire to the starter. I'm a little unsure if I will burn up the car with one hot to starter. The 2nd hot to the cars electronics, sc

[Ryland]

i think you should keep them both hooked together, the starting battery can handle the higher discharge rate that the starter motor draws so as the starter cranks over it's voltage is going to stay higher but the deep cycle battery should have more bulk capacity to it to sustain electrical needs, working together you get the greatest capacity as well.
If you wanted tho, you could just hook the starting battery to the starter and the deep cycle battery to the rest of the car electrical and use an isolation diode to allow the deep cycle battery to bump that voltage of the starter battery back up after starting, it would also allow it to "help out" a little with starting, that way the starter will always be sure to start even if you leave your lights on and a simple jumper wire would allow you to get home, auto part stores sell isolation diodes for RV's.

As for charging, if you have them in parallel you are fine to charge them together, 1.5 amps is a slow charge rate, small starter batteries tend to be around 40 amp hours and 120amp hours is not uncommon in larger engines and deep cycle batteries tend to be 100 amp hours on up to 225 amp hours, so if you are draining these batteries down, you could have 140 and up to over 350 amp hours of battery that you are trying to charge, at 350 amp hours you could pretty much leave that 1.5 amp battery charger on for weeks without worry of over charging and they tend to taper off as the battery gets full so it's rare that it's ever a full 1.5 amps unless the battery is dead, a good idea is to get your your volt meter and see what the battery voltage is after you pull the charger off the batteries, make sure you are getting them charged up all the way or you will kill your batteries early, for this reason I like higher amp battery chargers that either float or shut off when full, there are some nice weather tight ones that you can mount right on the car.

Other thing is, if you want both batteries to get the full advantage of the charger, hook the positive of the charger up to the positive on one of the batteries and the negative up to the negative on the 2nd battery, that way a bad connection between the batteries don't leave one lagging behind for an early death.

[SmellyCat]

Thx for the info. I was doing well this week. Mon, tue, on Wed. It rained. I used wipers and running lights. ran out of juice and had to plug exciter wire back to Alernator and get a jump start. This weekend I will Add Alt switch
And try again next week running on battery and only charging when the car is at freeway speed. Sc

[shovel]

Originally Posted by SmellyCat

Thx for the info. I was doing well this week. Mon, tue, on Wed. It rained. I used wipers and running lights. ran out of juice and had to plug exciter wire back to Alernator and get a jump start. This weekend I will Add Alt switch
And try again next week running on battery and only charging when the car is at freeway speed. Sc

Hook the alternator up to your brake light circuit, or maybe a mercury switch tilted to engage on downhill and hard deceleration..

[brucepick]

True that you can hook both batteries in parallel to the charger.

But if you run them down low your overnight charge will only charge them up a small fraction of what they can hold. Beware. Beg, borrow or steal a charger rated at least 10A, and 20A or more would be nice.

Since you're using a 1.5A charger it will provide only about 0.75A to EACH battery and each will take approx forever to charge at that rate. A car battery of average size will hold something like 40-60 Amp Hours (40 amps current for an hour, or 1 amp for 40 hours, etc). So 1.5A of current trying to charge up approx 100AH total of battery capacity (if both are significantly low) would be about 65 hours of charging!

Sorry I don't recall how far you're driving daily. My Civic uses about 10A if not using fan or headlights. Headlights adds approx another 12 amps. So a one hour drive with headlights is about 22 amp hours. Two hours drive with headlights is approx 44 AH. After one hour's drive (22AH), if I recharge the battery with a 1.5A charger, would take 14.7 hours. So I use a 10A charger instead, and let it run on a timer at night.

Related:
Not ideal to run batteries in parallel.
If you read up on batteries on RV sites, you'll learn this.

What happens is that the battery with more charge will flow charge into the other one. That way they both end up with only a partial charge. And there are inefficiencies in that charge going back and forth between the batteries, so you always end up losing a certain amount of the charge you had.

Consider doing what I did, or a variation on it.
My front (starter) battery has the positive terminal wired only to the starter. That's all. My deep cycle battery positive terminal is wired to the main fuse box terminal that provides current to the rest of the car. I charge each battery separately so the charger knows when THAT battery is fully charged, and it goes into its "float" or "maintenance" mode.