Charging Batteries in Parallel

[Sailing55]

Originally Posted by skipmac

Yes the controller should take care of the risk of overcharging but read the manual for your unit. There is also a downside to using diodes for isolating the different banks. Standard diode will drop 0.7V so you will lose a good bit of your charging voltage and slow the process a lot.

Sent from my SCH-I545 using Cruisers Sailing Forum mobile app

Thanks, I have my starting battery on position 1, and house batteries on position 2. Personally, I never use house batteries to start, and always let the starting battery come back to full voltage after starting.

My panels and wind charger are something of a mystery to me, so I always panic when someone brings up a topic that is in the realm of black magic!

[Sailing55]

Originally Posted by skipmac

This came up for some detailed discussion in a previous thread, with some quite detailed opinions posted by some very savvy people. While several experts recommended connections at opposite ends of the battery to compensate for resistance to equalize charging one came in a refuted this with some detailed data that basically the influence is minimal unless you are talking very high currents or very high resistance in the cables (too small or corroded connections). All things considered I think I agree that in a well designed and constructed system the charge difference between the first and last battery in a parallel chain fed from one end will not matter. However I still connected my + and - cables at opposite ends of my bank since it was just as easy and I figured why not.

I wired my 3 battery house bank the same way. After some research I decided the difference was too small to justify connecting [a half mile or so of] extra wires. For three batteries it is clean and simple.

[TranceNT]

Hamburking has a good point which is that storage batteries should only be connected in parallel if they are on a constant maintenance charge (float), or if they are normally off-charge, but are on a daily charge schedule.
Some heavy transport vehicles use paralleled starting batteries and this is OK as long as the truck is used on a daily basis.

The reasons include slightly differing effective terminal voltages, temperature differences (even quite small values). Terminal corrosion films etc. can also start the loop discharge.

The sequence is usually a chase or spiral to a discharged state as first one then the other effectively supplies current to the other -- and that is if both are nominally matched in age and specification.

If the group has one or more units which are not the same age, the problem is more pronounced.

Most Lithium-ion laptop batteries are comprised of a S3/2P 6cell arrangement -- three pairs of tightly paralleled 3.7 V cells (18650) in series to increase the "Watt/hr" capacity of the nominal 11.1 package. These ususally self discharge one or more of the three sets even while the battery unit is removed from the computer. However, very high quality units self discharge only slowly due to close tolerances in their manufacture.

Bottom line is paralleling for starting OK, but separate unless being charged.

[Sailing55]

Originally Posted by TranceNT

Hamburking has a good point which is that storage batteries should only be connected in parallel if they are on a constant maintenance charge (float), or if they are normally off-charge, but are on a daily charge schedule.
Some heavy transport vehicles use paralleled starting batteries and this is OK as long as the truck is used on a daily basis.

The reasons include slightly differing effective terminal voltages, temperature differences (even quite small values). Terminal corrosion films etc. can also start the loop discharge.

The sequence is usually a chase or spiral to a discharged state as first one then the other effectively supplies current to the other -- and that is if both are nominally matched in age and specification.

If the group has one or more units which are not the same age, the problem is more pronounced.

Most Lithium-ion laptop batteries are comprised of a S3/2P 6cell arrangement -- three pairs of tightly paralleled 3.7 V cells (18650) in series to increase the "Watt/hr" capacity of the nominal 11.1 package. These ususally self discharge one or more of the three sets even while the battery unit is removed from the computer. However, very high quality units self discharge only slowly due to close tolerances in their manufacture.

Bottom line is paralleling for starting OK, but separate unless being charged.

I had three group 27 batteries in parallel for starting a big engine years ago and they sat for several months at a time without being charged. They would not start the engine or take a charge at one point so I had them checked by a battery rebuilder and one of them failed the load test. He explained that if one battery is weak or sulfated from sitting it will discharge the other batteries. Or like you mentioned even if the batteries are of different ages or number if cycles.