Batteries - Diesel Forum

444-4D · 10-06-2009, 02:58 PM

I occasionally see a discussion about running different age or different size batteries in a dual battery application such as a diesel pickup. What's the real story about doing this?

drcampbell · 10-06-2009, 03:39 PM

To prevent confusion, let me first remind you that there are two different kinds of dual-battery installations:

A) One starting battery and one deep-cycle battery, as in RVs.

B) Two starting batteries wired in parallel to provide more current, as in trucks with large Diesel engines.

In RV installations, there's absolutely no reason for the batteries to be matched and they're usually not: One's designed for cold-cranking Amps and the other's designed for reserve capacity. A battery isolator is installed so they may be charged and discharged independently, the independence of the two batteries (more to the point, independence of the charge in each battery) being the primary reason for this type of installation.

In parallel-connected battery banks, the batteries need to be the same voltage and chemistry - no mixing lead-acid with nickel-cadmium - but there's no compelling reason for them to be the same size or age.

If they are at different states-of-charge when installed, they will equalize with each other after a few operating cycles. There will be little or no charge transfer from the full battery to the empty one unless it's near 90% depth-of-discharge; the terminal voltage of a fully-charged battery (near 12.6 Volts) just isn't high enough to make any significant amount of charging current flow from the fully-charged battery into the partially-depleted battery..

If they're different ages or sizes, they will share the load like good Marxists - from each according to his ability. They'll share the charge current the same way - the stronger/newer/larger battery will take a larger fraction of the charge current. When batteries are wired in parallel, their terminal voltages will necessarily be the same, and whatever the terminal voltage, they'll each respond with the same percentage of their individual CCA capacity. (once they're at the same state of charge)

The downside to installing two batteries of different capacities is that the pair might not provide enough capacity.

There is one good reason for replacing both batteries at the same time: If they were installed as a pair and nothing went wrong, they will have grown old and weak as a pair.

444-4D · 10-06-2009, 05:05 PM

Would a Motorcraft OE battery and an Odyssey AGM battery be considered the same chemistry?
Odyssey claims pure lead and Motorcraft claims lead alloy.

bryanl · 10-06-2009, 07:08 PM

That's one of the best summaries I have seen! A lot of folks get really hung up on battery matching and have to get into obscure failure modes to rationalize their feelings. Some who don't seem to care do just fine so it can't be too big a deal to parallel similar batteries.

I do think it best to at least keep to the same type as there are minor differences in charging and maintenance characteristics and internal resistance might make a difference in overall longevity and service life. i.e. as a first goal, keep to wet cells, sealed flooded, AGM's, or gels as sets and avoid mixing these. As a second priority, try to keep to similar size and current capabilities (CCA's). Then, for a third, go for same make and model. If you are really into it, go for close manufacturing dates.

I wouldn't mix an Odyssey AGM with anything other than a similar Odyssey, though, as a routine thing. They have some very interesting characteristics that tend to set them apart (e.g. Peukert well below most other batteries, including most AGM's).

Batteries aren't really that expensive these days for routine needs. It is usually good insurance to replace a set if you start having a problem.

drcampbell · 10-07-2009, 03:41 PM

I don't think the Peukert Number is applicable to this question. Peukert's "Law" is a correlation between the rate of discharge and the energy available. The relevant correlation here would be between terminal voltage and battery current. (in a parallel arrangement, each battery's terminal voltage will be the same as the other)

A problem might occur if the same terminal voltage resulted in two radically different currents being delivered from each battery, but don't think that will happen.

As for mixing "pure lead" batteries with "lead alloy" batteries, what's essential is which elements are taking part in the electrochemical reaction.

My understanding is that lead alloy plates are alloyed with another metal (usually antimony, aka. "hard lead") for structural strength, not electrochemical properties.

Using hard lead, battery plates can be made thinner, cheaper and lighter but with sufficient strength and the same amount of active surface area. The amount of surface area determines CCA performance.