Charging of parallel batteries

"Gents, what is the best way t

"Gents, what is the best way to charge two batteries in parallel...Don't want to burn the boat down..It's in storage now,,,winterized motor and everything is ready for Santa...Just the batteries..I will get a battery monitoring charger but they charge one at a time...How can I charge two at the same time???"

"If this is just for winter ba

"If this is just for winter battery maintenance, I would simply install a jumper wire between the batteries to connect them together. Just don't forget to remove it in the spring."

"Disconnect both negative batt

"Disconnect both negative battery cables, trickle charge for maybe 48 hours, and don't worry til spring!!!!!"

"I had them both fully charged

"I had them both fully charged,,can I leave them like this until April or should I put on a charger to maintain the batteries???"

"Is the boat left in the water

"Is the boat left in the water for the winter? If not why leave the batteries in the boat? I like to charge my two batteries seperately once a month to a float charge of 13.4-13.6 volts. Uncharged batteries can freeze and crack. If a charger goes bad and fails to regulate you can overheat the battery resulting in bloating, venting and possible explosion. Watch them carefully."

"Oh. One more thing. There a

"Oh. One more thing. There are true dual output chargers that independently charge two batteries. Xantrex is one. Quickcharge another. When charge two batteries in parallel with a single battery charger, if one battery has a bad cell the charger will be overcharging the good battery while trying to recover the bad battery. Not good."

"RE: "install a jumper wir

"RE: "install a jumper wire between the batteries to connect them together"

This is not generally a good idea. While you have it in your mind that the charger would be supplying current to both batteries more or less equally, the reality of electronics is that current always flows "downhill" and as fast as it can. How this equates to reality in your situation, is that unless both of your batteries are the exact same make and model and of the exact same age and state of charge, then the higher voltage battery of the pair will "charge" the lower one at a rate only limited by the resistance of the jumper wire. If you feel that you MUST do this (keep both batteries on a trickle charge all winter), then buy a battery isolator and charge the two batteries via that device. While charging a battery via an isolator does not result in a 100% charge, the difference in charge level on a "maintaining" charge is minimal.

In like manner, the common practice of boat owners who have battery switches with 1-2-BOTH capability should NEVER have the switch in the BOTH position unless they are in dire straites and this is the only way they can start the engine. Given the heavy cables and low resistance of the battery switch contacts, if there is a difference in the state of charge of the two batteries, a heavy and unregulated current can flow between the two batteries that could warp the plates in the battery.

BTW... one remote but entirely possible scenario of connecting the two batteries together and charging them.... Battery A fails with a shorted cell 2 am one Sunday. Voltage from Battery B flows via the jumper to Battery A. Since there is nothing to limit the current, the jumper wire overheats from the current flow, the insulation melts and the overheated wire starts a fire (since the charger can't regulate the current flow between the good and failed battery). When I was a curious kid, I used to routinely put wires across the terminals of 1.5 volt cells (lantern batteries), just to see them turn red, then white and then burn out. I used bare wires to do this since I learned that the smell of burning insulation attracted my parents attention "

"The boat is in a garage in AZ

"The boat is in a garage in AZ,,so not very cold there..I guess the best thing is to pull the batters and charge them once a month.."

No compelling reason to pull t

No compelling reason to pull the batteries. Just leave the battery selector switch in "off"

Onboard chargers are the best

Onboard chargers are the best bet for charging multiple batteries. You can get one at Bass Pro Shops or even move up to the Charles Chargers. I installed a 120v plug on the back of the boat and all I have to do is plug in an extension cord on the swim platform and it will charge all three batteries and monitor them.

"Bert, you do not mention the

"Bert, you do not mention the size of the boat nor your current battery bank configuration.
Do you have an MBSS (main battery selector switch)?
Do you have multiple banks?

I'd not recommend to anyone the use of a trickle charger unless you can be there to take a reading and disconnect when fully charged.

The proper method would be to use an true Marine O/B, three stage, multi-bank charger with "Smart-Charge" technology only.
Assuming that you have two battery banks (1 cranking battery [to be held in reserve]..... and 1 bank of deep cycle batteries) I'd connect each charge lead to each bank individually so that the charger can "Sense" each bank independently.
This charger lead connection is best accomplished by using the rear terminals (# 1 & # 2) of your MBSS for the charger lead connections. This provides a permanent connection and keeps the typical "Rat's Nest" of wiring from any direct battery connection.
Schematically, this is the same as a direct battery connection.... but avoids the small cable corrosion that can and will exist.
I do only main cables at the batteries themselves!
Set the MBSS to OFF or # 1 or # 2 ....... (never BOTH/ALL) ....... when using this O/B charger!


The above is the correct way to do this, IMO."