These two questions are close to what I'm asking, but I'd like some specific clarifications:

How do we charge a small 12V battery from a large 12V battery bank

Increase the capacity of a 12v battery bank by adding new batteries

So, I have a sort of rag tag 12v Deep Cycle battery bank that is normally on a trickle charge. It contains a 56Ah (2yrs old) and a 35Ah (new) as the two main batteries that are not removed. I also have a couple of smaller batteries that are trickling with the mains most of the year. They are a 4.5Ah and 12Ah. This entire bank is used to power my telescope a few times a year. When we go to Burning Man, the two smaller batteries power the lights for our two bikes at night.

Through the course of the night these smaller batteries are discharged a fair amount, but probably not below 50%. The mains stay put and power a small fan that runs 24/7 and the LEDs for interior lighting. The mains may only discharge at most 20% over the course of a given evening. The 35Ah is new (replacing an worn out 18Ah) and hasn't really been properly cycled yet.

So, before we turn in for the night, I take the smaller batteries off of the bikes and plug them right back directly into the battery bank. No circuits or anything, just straight wire to wire contact (I'm using power poles and power pole splitters).

In the morning, a 30w solar panel and 4.5A charge controller come "online" and begin to charge the entire system. After a few hours of daylight, I'll turn on a small AC inverter and charge cameras, 18650s and phones (mostly the 18650s as they power other night time LEDs). All 12v batteries are just shuffling around whatever charge to wherever it's needed. I've never really analyzed the system outside of how discharged everything gets, and how much energy is coming from the solar panel during the day; mostly so I can move the panel to get the best angle. I've done this in the past years and it seemed to work, but I'm curious about the science behind "shuffling energy around wherever, whenever".

My question is if this is good, bad, or could be better? Is it bad to have a smaller battery at a lower voltage suddenly directly connected to a larger battery at a higher voltage bad?

  • \$\begingroup\$ Bad idea. You should charge it first. \$\endgroup\$
    – SDsolar
    Sep 7, 2017 at 17:54

1 Answer 1


Yes, it is a bad idea.

Manufactures of the battery usually prescribe the maximum initial charge current in their datasheet. A typical lead acid battery has maximum initial current 0.3 CA or 0.4 CA for cyclic charging, where C is the rated capacity of the battery. For example, a 4.5 Ah battery with a maximum charging current 0.4 CA can be charged with 1.8A. If the current above this level, the battery can be damaged.

In your case, we do not know how much the current will flow from the higher voltage battery to the lower voltage battery. The higher voltage battery would charge the lower voltage in an uncontrolled condition.

The simplest solution is you can add a resistor to limit the initial current. For example, if the lower voltage battery is 11 Volt and the higher voltage battery is 13.8 Volt, you can use 2 Ohm resistor / 5 Watt to limit the initial charging current to 1.4 A. After several minutes or hours, when the voltage difference is not too far (say maximum 0.1v), two or more batteries can be safely connected in parallel without resistor.


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