OT: Charging two batteries in parallel

crossthread · Nov 10, 2020

I have two 12volt batteries on my sailboat. They are connected through a three position switch which can connect battery A , battery B or both to the distribution panel. When I got the boat there were two trickle/maintenance chargers with one on each battery. One of them has died. My question is, can I put the switch in the "both" position which will put the two batteries in parallel and charge them with one charger? My concern is that if one battery is nearly dead and I parallel it with a possibly fully charged battery then the battery and or the charger may get damaged. Thanks for any replies.

gustafson · Nov 10, 2020

that is the risk with any sort of interconnected batteries, but no worse, i think than them just sitting there

woodsrider845 · Nov 10, 2020

Electricity is like water, it will seek it's own level. So, if one battery is at nominal 12.6 v, and the other is 8 v, connecting them in parallel would result in a voltage of 10.3 volts. That said, the benefit of parallel wiring batteries is doubling (in the case of same rated batteries) amperage capacity. Series wiring would double voltage (in same rated batteries).
There is no harm in charging dual (parallel) connected batteries, as long as both batteries are in proper working order. It is the same as charging a battery with twice the current capacity.

Strostkovy · Nov 10, 2020

For lead acid batteries it's not really an issue. Other chemistry types may or may not need to be balance prior.

My concern with the system is that any time you switch to "both" with one battery at full charge and one nearly depleted you will have significant current flowing from one battery to the other. Not great, but not a huge deal. Worse than charging in parallel though.

NC Rick · Nov 10, 2020

I'm using two 100 ah batteries in our van which are charged by solar or shore power and it works fine, always in parallel. On the boat we had, the two batteries were charged by a single constant voltage ferro-resonant charger but I was always having problems. Today's modern chargers are really efficient. If you can look at a company like C-tek for the two channel battery charger/maintainer which I think will pay for it's self in battery life. My 0.02

mattthemuppet · Nov 10, 2020

if you connect batteries in parallel then current will flow from the higher voltage one to the lower voltage battery until they're the same voltage. Only possible issue is if the voltage difference is large and the current capacity (or C rate really) of the batteries is large then that current can be very large.

CarbideBob · Nov 10, 2020

In the both position if one dead one up the bigger is simply a jumper battery as if a car starting yours.
Top guy feeds the other. Given time,,, equals.
Charging and you are now feeding both so eat that power as needed.
Two trickles done here as to keep a reverse. Not good to run off both at sea.

Bob

PDW · Nov 10, 2020

crossthread said:
I have two 12volt batteries on my sailboat. They are connected through a three position switch which can connect battery A , battery B or both to the distribution panel. When I got the boat there were two trickle/maintenance chargers with one on each battery. One of them has died. My question is, can I put the switch in the "both" position which will put the two batteries in parallel and charge them with one charger? My concern is that if one battery is nearly dead and I parallel it with a possibly fully charged battery then the battery and or the charger may get damaged. Thanks for any replies.

My boat is set up with the alternator charging the start battery and a DC-DC charger charges the house bank only when the engine is running. I never parallel my house and start batteries.

The charger I use is a Redarc 40A unit which also can take a feed from PV panels. There's a solenoid that flips when the engine is running. Engine off, charging from PV panels.

3rd season in the water, all batteries fully charged and happy. Now I'm going to complicate it by adding a powered anchor winch, so will increase start battery size and run the winch off of it. Very unlikely I'd use the winch without the engine running.

PDW

gregoryd · Nov 10, 2020

I have a truck and a van that have 2 batteries (extra capacity for running the plow and wheelchair lift). I overthought the same question for a while and then just did it - that is use one trickle charger in the circuit for both. It has not caused me any problems in the years that I've done it.

Laverda · Nov 10, 2020

My Chevy pickup with a diesel has two 12 volt batteries in parallel from the factory. As it is not driven much it has one charger on all the time for both batteries. First set of batteries lasted 11 years with this set up.

Tony Quiring · Nov 10, 2020

If both same condition fine but a bad battery will effect good one.

Parallel batteries common and single point charging too.

But if mixed batteries then you can add a diode isolator between charger and batteries, this removes about 0.6 volts from charging voltage but isolates the batteries from each other.

Sent from my SAMSUNG-SM-G930A using Tapatalk

CarbideBob · Nov 10, 2020

PDW said:
...

The charger I use is a Redarc 40A unit which also can take a feed from PV panels. There's a solenoid that flips when the engine is running. Engine off, charging from PV panels.

PDW

You have panels on the boat?
What happen to the old days of sails, no motor, no electricity .
Is this sissy life? Gonna guess GPS and all that too?
That and you call yourself a sailor.
Bob

Phil in Montana · Nov 11, 2020

As many have said it works good if both battery's are the same but if they are different or older/newer not so much, you can use a diode split-er and solve this problem but you need to raise the charge voltage .7 volts for the voltage drop in the diodes...Phil

Scottl · Nov 11, 2020

What I would do is pull the negative on one battery while charging the other as a temporary workaround.

Long term a replacement charger would be the safest bet.

EmGo · Nov 11, 2020

All the modern marine chargers I have seen have separate leads to each battery, so the one-two-both switch doesn't matter, for charging. The charger itself determines how much to charge each battery, individually. Then your switch is just used for loads.

While we're here, you should also have a "galvanic isolator" between your shore power and the boat electrical system. That can help assure that your charging system doesn't electrolyse your fasteners away while Ms Boat is sleeping.

crossthread · Nov 11, 2020

Well Bob, calling yourself a sailor is kind of relative these days. My last sailboat was a gaff rigged 38' Chesapeake dead rise built in the 40's. No motor, no electricity (had kerosene running lights). I got to where I could back it into a slip using the working jib so I guess I am at least part sailor. As for the sissy part, I've been de-masted twice which makes me an automatic non sissy. Stupid maybe, but certainly not sissy when it comes to blue water sailing.

rat_stomper · Nov 11, 2020

You could use a diode isolator to charge both batteries while preventing them from discharging into each other.

I did it on my buddy's boat so that the "cranking" battery couldn't discharge into the "accessory" battery.

crossthread · Nov 11, 2020

I thank everyone for the replies. It sounds like I can charge both off one charger as long as there is no huge delta between the battery voltages. I like the diode idea but as mentioned, I would have to up the voltage on the charger which I don't think I can do. Emanuel, I've got a couple of zinks here and there and hopefully that will keep things from dissolving. I think I am going to leave the switch in the "both" position and try using one charger. When I am running I will of course either run off the "A" battery or the "B" battery but not both so I will have a reserve as Bob mentioned. I could always get another charger but it is just one more thing to crap out (harsh environment). The only way I knew the charger was bad was that one of my batteries was dead and the other one was fine. It just seems like it would be easier to monitor things if I just had one charger.

EmGo · Nov 11, 2020

crossthread said:
I thank everyone for the replies. It sounds like I can charge both off one charger as long as there is no huge delta between the battery voltages.

Until a cell shorts out in one and the other dumps all its stored electricity into the first one, causing a nice onboard fire while you are home sleeping. You should always isolate the batteries.

Emanuel, I've got a couple of zinks here and there and hopefully that will keep things from dissolving.

If it's wood, zincs can make the situaton even worse. Overzincing a wood boat will eat up the wood around the fasteners. Planks are harder to replace than screws.

Otherwise a small, continuous, voltage leak either from you or from anyone else in the marina, since you share a common ground, will eat those zincs up lickety-split. Should have an isolator on the ground lead to stop that. There's a guy on craigslist who makes nice ones for $60, where West Marine charges $300. Surprise surprise, I know

It just seems like it would be easier to monitor things if I just had one charger.

A proper, modern charger will take care of that. Like I said, they have separate leads to each battery, and control the rate of charge individually and also isolate each battery. Makes the batteries last longer, too. Trickle chargers are not as good, either for battery life or safety.

Strostkovy · Nov 11, 2020

Is there ever a reason to use just one battery at a time? If the batteries can be in parallel forever that is going to be better for them.

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