Charging Dual Batteries

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?

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.

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

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

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.

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.

https://www.googleadservices.com/pagead ... EAs&adurl=

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-cha ... -parallel/

Aye.

Foggy":c0ga65k9 said:

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

https://himaxelectronics.com/how-to-cha ... -parallel/

Click to expand...

Good advise and link Foggy. tight/clean/dry - as in bare metal to metal contact between post and cable connection. Never use dialectric compound on post or connector before attaching and torquing. It is designed specifically to resist the flow of electricity. In particular on low voltage/high amperage systems(12v) the increased resistance can cause a fire. I prefer using battery terminal spray - after all connections have been made - think it is made by CRC?

Rob

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.

Thanks for all the feedback! My batteries are the same age/size/type. What I've decided to do is charge each to full capacity independently (Perko OFF), then hook up the charger to them "both" (Perko set to ALL) while storing for the winter to keep them maintained. Sound good!?

But here's what I don't understand. How do batteries get overcharged? For example, I'm running one day with the Perko switch set to "All" (as recommended while the engine is running). Then that night the engine is off, and I'm using the house battery and then the next morning it's somewhat discharged.. And then the next day I turn the Perko switch to "All" while the engine is running, how does the alternator NOT overcharge the fully charged battery?

My understanding: If the 2 batteries are identical except for a (small ?) voltage difference and are connected properly in parallel, the more fully charged battery will "backfeed" amps to the lesser charged battery until the voltage equalizes. During charging (see above 18 Feb 2025 post), the same process occurs. Potential for "burn out" is less if the starting voltage differences are not large. And modern multi-stage chargers "sense" when to decrease then end the charge cycle.

Aye.

Foggy said:

My understanding: If the 2 batteries are identical except for a (small ?) voltage difference and are connected properly in parallel, the more fully charged battery will "backfeed" amps to the lesser charged battery until the voltage equalizes. During charging (see above 18 Feb 2025 post), the same process occurs. Potential for "burn out" is less if the starting voltage differences are not large. And modern multi-stage chargers "sense" when to decrease then end the charge cycle.

Aye.

Click to expand...

Yes, this raises the question, how/when does a charger know to stop charging. Does it just read the battery voltage and use that information? Or does a charged battery start to "push back" at some point (as you suggested, with the more-charged battery then sending juice to the less charged one)?

That's an engineering question. I'm not an engineer.
You most likely can come close to getting your answer my digging into articles about 12V battery chargers; Practical Sailor, Passagemaker, and the like.

Aye.

As Thataway said, a starting battery and deep discharge (house) battery should not be charged in parallel (both at the same time). So, I assume this question relates to two of the same batteries in parallel. If there is a slight difference in voltage between the two, power will flow from the battery with the higher voltage to the one with the lower until they are the same, even when not charging. So, when you start charging the voltages should be approximately the same. If the batteries are the same they will charge at the same rate and be full at the same time. The charger will sense when the max voltage is reached and shut down. With lead acid batteries a charger will then go into a float mode and apply a small current at the max voltage to maintain the max. This will also help equalize any slight differences between the batteries.

Lithium batteries however, are different. They have an internal BMS (battery management system) that shuts charging down when the internal voltage reaches a certain value. If you have two lithium batteries in parallel and the voltage shut-off for the charger is higher than the BMS shutoff then it will keep charging the other battery that is at a lower voltage. However, if the voltage shut-off in the charger is lower or equal to the one in the BMS the charger will shut off before the second battery is fully charged. You will then have to rely on the "voltage balancing" between the two batteries to equalize the charge.

technically no, you do not want to set the battery to "both" all the time. That is for emergency use. If the house battery is low V from running a fan or heater all night and the starting battery is high V from being charged the entire trip to your anchor spot- starting the next morning, it can over charge the starting battery. You'd need the Add a battery kit from Blue Sea to do it "correctly". Do many people set it to both and let the batteries come to an State OF Charge equilibrium without issues? yes.

What you are asking is something, I’ve been doing similar to for years with my twins, when running one motor during displacement speed cruising & setting the switch to all, combining both my batteries & have the single running motor charge both batteries at the same time, while both batteries are used as house & start.

I’ve been running twins with the 1,2 & all switch & 2 identical batteries now for 23 years. The batteries, after the original set have all been AGM dual purpose & for me both are used as such. When running both motors on plane, I will set whatever single battery is at the highest charge as the house battery. At displacement speed where, I normally only run one motor, they are periodically switched, while the batteries are left combined, unless one has a significantly less charge. The batteries are always combined if using the windless or when starting the Wallas stove with the motors off.

I don’t have to be near as watchful about the batteries charge state & their switch selection with the replacement twin Honda 60’s as they have more than double, the alternator output over the old Honda 40’s. Also with the 1999 Honda 40’s if the batteries were combined with both motors running & sea state or other causes created excessive throttle adjustments, this could cause problems with the coil charge electrical grid. On two different extended SE Alaska cruises, I burned out a charging coil due to this. I’ve read this problem has been corrected on the newer Honda motors, but don’t know for sure if it has, so I’m still very cautious about combining the batteries when running both motors.

I realize the simple 1,2 & all switch by itself is outdated compared to what’s available & one has to put extra consideration into individual battery state of charge, but so far, I prefer it’s simplicity combined with my lower overall house power use, than the newer more advanced monitoring systems. All my batteries thus far have lasted a minimum of 6 years before replacing, so after 23 years of not having to replace batteries due to failure, while cruising, I have no desire to change the system or my mode of operation at present.

Jay

Both my batteries are FLA from Walmart and are the same size and dual use. I haven't really thought twice about what the battery switch is set to, just which battery needs charging. Typically one battery is used and one is reserve. After a long night anchored out, I may set the battery switch to Both to start the engine (just in case) the next morning, depending the used battery SOC. This is mostly to protect the electronics from the voltage sag when starting on one partially charged battery. If I do that, then I leave the battery switch to Both in order to charge up both batteries, then switch to 1 or 2. I haven't had any issues doing this and my batteries last at least 5-6 years.

FWIW, I wish battery switches had outputs for remote tell-tale lights. My battery switch is in the aft lazerette and cannot be seen without opening the hatch. Having indicators on the electrical panel or helm showing which battery was selected would be nice.