Charging Dual Batteries

[Wood Zeppelin]

I was just thinking... If I put my charger on one of the batteries, and then set the Perko switch to "all" (both batteries).. will I be charging both at the same time!?

If so, is it ok to do this?

[thataway]

Yes and yes. It is ideal to charge each battery separately, especially if they not the same brand, size or age. Generally one does not mix battery types, such as LiFePO4 and flooded lead acid.

[Robert H. Wilkinson]

As Bob has pointed out - not advised unless both are the same type(start - deep cycle - FLA, etc). Both the same age(condition). Both the same size(AH capacity).

In addition I would point out they must both be at the same state of charge. For example never charge your start battery(which will likely be near full charge) in the same circuit as your house battery( that was drawn down at anchor overnight and may be below 50%). The charging source will only recognise their combined voltage and your house battery would be undercharged and your start battery overcharged. This can lead to 1 or both batteries early failure.

I solved this problem by charging my house battery from the alternator and topping up my start battery with my onboard charger through the inverter. You can also install a system that charges 1 until full then automatically switches to charging the 2nd battery.

Rob

[ssobol]

Yes. I do it regularly and my batteries last a long time. But you should have the same type and size for both batteries.

[Tom Hruby]

Charging two batteries simultaneously only works if they are the same brand, same chemistry, same voltage, same amp-hours and same state of charge. Any other combination will result in unequal charging and potential damage to your batteries.

Even if batteries are identical they will still need to be rebalanced occasionally since each has a slightly different internal resistance and charge/discharge behavior. One way to do this is to disconnect everything and then connect them in parallel (+to+ and - to -) for 24 -48 hrs.

As mentioned, starting batteries and deep discharge batteries should never be connected together, either in series or parallel.

[kennharriet]

A fellow Cbrat suggested using a combiner several years ago and it works great. I use the Yardina Combiner 100 for my all AGM start and house battery bank. It prioritizes the charging of the start battery until topped of, then switches the charge to the house bank. No need to remember to switch between batteries. Easy install. Yardina makes another model for lithium batteries.

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[Foggy]

Below is a step-to-step guide to charging 2 12V batteries in parallel.

I've used this method to charge 3 12V AGM deep cycle batteries parallel;
good but not perfect for more than 2 batteries.

If you dig into this charging method for more than 2 paralleled batteries,
there are other connection methods to provide a more even charge to each.

And remember, the key to all electrical connections is tight/clean/dry.

https://himaxelectronics.com/how-to-charge-two-12-volt-batteries-in-parallel/

Aye.

[Robert H. Wilkinson]

[T.R. Bauer]

As others pointed out, it's generally fine if the batteries are the same size and chemistry. They are correct it's not a good practice to mix the chemistry. But there is a mountain of anecdotal examples out there going against the grain...Think of how many RVs are out there with AGMs as house batteries....While they are driving down the road many of those batteries are being charged in parallel with the starting battery without any isolation or other protections in place. Not advised....but apparently it is a minor infraction...probably best to follow the rules.