I have done quite a bit of research, here and elsewhere but am unable to come with satisfactory answers to some questions I have.
I have a simple dual battery set-up in my car, with a ‘dumb’ solenoid to isolate one of the batteries when the ignition is turned off. (will be running a 32L Engle for 2-3days using the isolated battery).
I am looking at battery options for the second battery, and by all accounts an AGM battery is the best way to go.
I spoke to a proprietor of one of the large 4wd accessory chains the other day and he said that I can not install a AGM with a traditional wet cell lead acid battery as my primary – at least not without installing a ‘smart’ charger which simply senses voltage levels to determine which battery to charge. He explained that if he sold me the AGM, he would not offer a warranty if installed in a dual battery system where the other battery is a traditional wet cell. My main battery was manufactured less than year ago and I don’t really want to shell out on buying two AGMs!
When asked why the proprietor then started to mumble stuff about differing internal resistance (true), but said nothing that really explained why mixing batteries will lead to premature failure of the AGM. He was happy to sell me either two AGMs, a marine battery, or a single AGM with smart (dumb?) charger.
Anyway, why are we told not to mix battery types and sizes?
I accept that mixing battery types is bad if and when one fails, it can cause the other one to fail prematurely. By keeping the batteries of the same type and age, you reduce the risk of a dud battery ruining the other good battery. But until the first failure, why would a dual battery system accelerate the failure of the first battery?
Another argument commonly put forward is that the alternator does not correctly charge both batteries, particularly if they are of different kinds. I am not sure about this, and this is why…
As far as I understand, an alternator (in most cars) does not really care, or know as such, what battery, or how many batteries are connected. All it really cares about is trying to maintain its preset voltage (about 13.9 -14.0V in my car), regardless of current draw - within the capacity limits of the alternator. It is the batteries own internal resistance and any other electrical accessories that determine how much current the alternator will put out. I understand that 3 stage battery chargers vary the current and voltage, but am not aware of any alternator car charging systems that do this.
So why would any one battery not be charged (as much as it can be at the preset voltage of the alternator) in a dual battery system, provided the alternator was operating correctly and of sufficent capacity?? (assuming the cabling is good, and with minimal voltage drop between the two batteries).
Of course, odd batteries will charge at different rates, and some may take longer, but this would be the case regardless of how many batteries are connected. I also understand that some battery types require a different charging profile (different voltage?) and this can cause problems if its ideal charging voltage is higher than what the alternator provides. However, AGMS are supposedly able to charge quite
well at relatively low voltage so this should not be a problem on my car.
Can someone give me a logical reason as to why installing a AGM and a traditional battery in a dual battery set up will result in one battery (particularly the AGM?) dying prematurely?
And if I install a AGM in parallel with a traditional battery, can the alternator fully charge both batteries?
The car is a 1995 Disco TDi, with a standard 100 or 120Amp alternator (can’t remember which).