Charging Two Batteries

[Ben]

I have a question about charging batteries. I need to charge two 12V batteries. If they are connected in parallel, can they be charged with a single 12V charger and have the automatic function of the charger perform properly?
Thanks for the help.
Ben

[nemo]

You can set them up so one charger can charge 2 batteries, but you don't want them directly connected together in parallel because then under use they will discharge together and you could find yourself running out of power and not being able to start your engine. You want them isolated during use so one is always charged for engine starting.

[Tom Spohn]

I agree with Nemo if you have an engine that cannot be hand started such as the newer larger engines common on the M model and some of the later X models. If, however, you have one of the older 2 strokes, like the Nissan/Tohatsu for example which can be hand started in a pinch, (try it first) then there are some advantages to leaving the batteries connected in parallel and using them as one large bank. The current withdrawn from the bank will represent a smaller percentage from a large bank (two batteries in parallel) than from a small bank (single battery) thus preserving the lifespan of the batteries. If you keep them linked the batteries should be of the same type.

Make sure your charger is one of the three stage chargers to insure best charging and battery life.

[Tahoe Jack]

Expect the board is roughly evenly split on this issue...I use the batteries in parallel, however, I have set up Perko switches so that I can reserve a battery or charge a single one if I want to. Re emergency start...I picked up a heavy duty jump start setup from Costco...about $65. It has cables, light, air compressor etc. so it serves several purposes and allows me to fully use my primary batteries if needed. Drawing two of them down 25% is easier on them than drawing one down 50% according to the battery folks.

[Andy26M]

I know my Honda 50 came with an emergency starting pull cord. I have never tested it out, but I have in the past had to help start my Dad's 115 Merc with the pull cord (it was a b*tch).

I suspect the 50 would not be too hard to start this way...has anyone tried it?

- AndyS

[Paul S]

If you use the 3 switch with VSR, you can have your cake and eat it too.

Both batteries are totally isolated from each other. As the starting battery charges..and finishes..it automatically charges the house battery (VSR - voltage sensing relay). You can just throw a battery charger on the starting battery...and in time it will charge and maintain both. Same with the motor..it will charge the starting first, then the house. When the charge is taken away..the relay opens and they are separate again.

If you NEED both batteries (to jump the starting, or for more amps, emergency..etc) just flip the emergency switch and combine them.

In this setup .. it is impossible to flatten both batteries if you leave a house accessory on ... (unless you jump the emergency switch)

Best part is there is no interaction with the switches on a daily basis.

Here is a pic of the BEP unit I installed:

[kmclemore]

A couple of points to add...

First, as Tom noted in part, both batteries must be of the same type, size and overall condition for this dual charging to work out properly.

Second, although there are safety advantages to running the batteries independently (i.e. keeping one for 'starting', and using the other for 'house'), in the average marine use you'll get a considerably longer power 'run time' if you use the batteries together as one... this is due to what is known as the "Peukert's effect" (Google it).

Third, not all engines can be pull started, even if they do have a rope. Many that have on-board electronics require some sort of minimum input power in order to start, so if you let both batteries go dead, you're engine's dead too. To overcome this, some folks keep one of those portable battery-powered jump starters on board.

[Catigale]

Ive practicied with my Mercury BF 50 HP EFI and it isnt too bad. You have to do the trick of turning it onto a compression stroke like a lawn mower but Im confident I can "onepull" start it.

On mine, you have to take the pulley off with tools to access the winder for the rope, so practice this at home ....and dont drop tools...

[Ben]

Thanks for the responses. I should have been more specific. These two batteries will be #'s 3 and 4 on my boat. They will be used to power a 24V trolling motor. When being used, they will be in series. I would prefer to use the 12V charger that I already have rather than spend another $100 to $200 on a new charger. It would just take a few minutes to switch them back to parallel when I get back to the marina. My charger is 10A initially then steps down to 2A for the final I think 25% of the charge and then changes to maintenance mode. I just wasn't sure if the three stages would work properly wiht two batteries. The batteries are new identical Die-Hard Marine units. I plan to mount them under the forward dinette seat.
Thanks Again
Ben

[kmclemore]

Well, Ben, it sounds like as long as you charge in parallel (not series) then your charger should do the job. But I'll let Moe confirm that, as he knows FAR more than me about electrics and electronics.

Ive practicied with my Mercury BF 50 HP EFI and it isnt too bad. You have to do the trick of turning it onto a compression stroke like a lawn mower but Im confident I can "onepull" start it.

Agree that you can hand start these engines fairly easily, but have you tried to start it with no battery power (i.e. simulating a 'dead' battery)?

[Moe]

Yes, if you rewire the batteries from series to parallel, you can attach the charger to them. I'd get in the habit of connecting it to the positive and negative terminals on the same battery when they're in parallel. That way, if you did accidentally connect the charger with them in series, you won't do any damage... just won't charge one battery.

--
Moe