I just installed a brand new deep cycle marine battery. It goes thru a charger-inverter and will switch over automatically if the power goes down.

My question is what to do with the other two (old) batteries. Both old batteries are deep cycle marine also. But one is 10 years old and the other is 20 years old.

Does any elmer anticipate any issue with hooking up the other two batteries in parallel with the new battery?


2 Answers 2


When you parallel batteries, the combined reliability generally becomes less than the least reliable battery. In reliability engineering, this follows the equation:

$$R_S=R_1*R_2*R_3 \tag 1$$

where $R_1, R_2,R_3$ is the reliability of the respective individual batteries.

The older batteries are inherently more unreliable. Battery failure modes can cause the charging system to not properly charge the individual batteries and it can reduce the available energy when a load demand is asserted.

The only failure mode that does not reduce the system reliability is a failure to an open condition. This is rarely seen on secondary batteries except when they contain integrated charge controllers, which is not the case for typical lead acid type batteries.

To exemplify the formula, imagine that the new battery is 95% reliable and that the old batteries are 50% reliable. The combined reliability is 0.95*0.5*0.5 or 23.8% compared to 95% for the new battery alone.

Aside from failures of the individual batteries, you may find that your charge controller is either not equipped to handle the combined charge current or it will not properly detect a charged condition of the combined batteries.

If you wish to make your system redundant, at least from the battery perspective, you need to design the system to reliably charge each battery and also allow for each battery to independently supply the load regardless of the state of the other batteries. The reliabilty of a such a redundant system is:

$$R_P=1-\frac{1}{\frac{1}{U_1}+\frac{1}{U_2}+\frac{1}{U_3}} \tag 2$$

where $U_1, U_2, U_3$ is the respective unreliability of each battery. Unreliability is defined as 1 minus reliability.

Using the same individual reliabilities from earlier, this would result in a combined battery reliability of ~96% - not really worth the effort given the marginal improvement over the 95% reliability of the new battery.

If you are relying on this battery system for any critical function, I would recommend against combining the batteries.


I'll bet those old batteries are just plain worthless.

Based on our experience with many deep-cycle golf cart and automotive batteries, those 10- and 20-year-old batteries almost certainly are not capable of accepting anywhere near a full charge. At best, some cells may work to some degree while other cells are just plain dead, Jim. ;-)

You can determine that for yourself by attempting to charge them (one at a time) and then see how long the battery voltage stays above 12.6 VDC under load. I think you'll find that they are completely worthless for any purpose and should be disposed of.

I had ~24 used 6 volt golf cart batteries in series to store electricity from a wind generator. Batteries constantly would fail, and so I finally gave the unfinished project to an uncle and moved on to other things.

  • $\begingroup$ They’re not entirely worthless. You can use them as doorstops or get a few pennies at a lead recycler. $\endgroup$
    – RoboKaren
    Oct 1, 2018 at 4:15
  • $\begingroup$ Sometimes, they can be brought back to life. In this YouTube video, material from the plates shorted out at least one cell. Rinsing it out like he did restored it enough to start his vehicle, but I wouldn't count on it on a cold winter morning! $\endgroup$ Oct 1, 2018 at 17:20

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