Thread: Crap "dual" battery system

Originally Posted by drivesafe

Hi Steve, I’ve never found any real problem with paralleling dissimilar batteries and we have been doing so for many years.

A couple of thing to consider though, standard deep cycle wet cell batteries are not designed for large current dischargers but they will still start a vehicle and doing so, once every blue moon, will not have any real detrimental effect on the life span of these batteries.

On the other hand, if you continuously use this type of battery for cranking, you are going to shorten it’s operating life.

In the case of you question, as you are sharing the load over two batteries, effects on a standard deep cycle wet cell battery is not going to be as hard so although you will probably still shorten its operating life span, this will probably be only a minimal difference and on the flip side, again because you are sharing the load, you will probably extend the operating life of the cranking battery.

In both cases though, the differences would only be marginal.

Another point to keep in mind when paralleling dissimilar batteries, they will most likely charge at different rates meaning it will take longer to charge one type of battery compered to the other.

This again is not a major problem, particularly if either you do a long drive after using them or if they have only had a small amount of their power used before starting to charge them.

If you do discharge them down a few bit and then drive for only and hour or so, when you stop the motor, there is a good chance that one battery will have a higher State of Charge ( SoC ) than the other.

What will happen in this situation is that the lower charged battery will simply burn off the higher battery’s SoC difference until the higher charged battery has the same SoC as the lower battery and this will not cause any problems, it will just be a waste of power.

No matter how high the higher battery’s SoC is or how low the lower battery’s SoC is, both batteries will eventually end up with the same SoC as the lowest battery and the lower charge battery WILL NOT gain any charge at all.

Cheers and hope this answers your question.

The $64 question is then how can you charge up a deep cycle battery quicker. The problem I had on my last trip was on days where we cranked the k's the deep cycle one only just got back to full charge but when we stopped for a few days at a "nice spot" the battery ran right down powering the fridge etc. Then it took heaps to get it back up again.

Can you fit a bigger alternator or some other way of getting the charge back in quicker.

You could fit solar chargers, or bin the A/C and fit dual alternators, or bin the standard alternator and shoehorn a big Bosch one in. A less permanent solution would be one of those small 2stroke battery chargers.

Originally Posted by Chenz

The $64 question is then how can you charge up a deep cycle battery quicker. The problem I had on my last trip was on days where we cranked the k's the deep cycle one only just got back to full charge but when we stopped for a few days at a "nice spot" the battery ran right down powering the fridge etc. Then it took heaps to get it back up again.

Can you fit a bigger alternator or some other way of getting the charge back in quicker.

Hi Chenz, the short answer is, to shorten the charging time and as such speed up the charging of any automotive battery, you need to increase the charge voltage.

Current is nowhere as much an issue as voltage is when it come to the time it takes to charge a battery.

Now the seemingly obvious thing to do is to raise your regulator voltage.

Surprisingly though, this usually is not the end all to the problem.

You need to start by measuring the voltage at your cranking battery, while the motor is running and you need to measure the voltage at your auxiliary battery.

The voltage at the auxiliary battery can vary greatly, depending on the State of Charge ( SoC ) of the auxiliary battery, the length of cable between the two batteries and probably the most important item, the thickness of the cable.

Anyone of the above can cause an unnecessary voltage drop that will have a major effect on how your dual battery system performs

These and bad earth returns cause most of the problems with auxiliary batteries not charging properly.

Of the above, using too thin a cable and/or not installing a negative cable probably accounts for about 90% of under performing dual battery system's problems.

Cheers.

like drivesafe says above, take accurate readings of the voltage going into the aux battery with stereo, a/c fan and fridge running. i'v installed a few duel battery systems for mates since i'm a sparky and the old man used to teach auto elec. i allways link the batteries with minimun 10mm 2 core double insulated flex when the battery is in the same vehicle, 16mm when the battery is in a trailer/van.
after the poxana bd 150 s(hit) died a coulpe of months ago and took out a exide orbital blue top ($320) i am constructing a voodoo doll of poxana parts that have died. they have offered to fix the controller for free but i will never trust it again, certainly not with a gel cell ($) connected to it.
david

Originally Posted by drivesafe

Hi Ian, any idea of what the setting is in your voltage sensing alarm is.

Cheers.

I don't know if the alarm triggers at a preset voltage. I think it operates more on the principal of a sudden voltage loss such as opening the door and the interior lights causing a voltage drop, however that does not work since I fitted led globes to the interior lights. The alarm is a Cobra brand.
I run 12 volt chargers regularly during day often dropping the second battery to quite low voltage. Also the alarm works the central locking and when I go camping and have the fridge running I suspect the fridge starting will set the alarm off if the batteries are not completely isolated. I may be wrong about the way the alarm works but I cant find the instructions, they are in the garage somewhere.
I have noticed recently that the alarm is of little use anyway. people just smash your windows, take what they want and everyone else just ignores the alarm. Probably better with just an ignition cut out so the whole car cant be taken!

thanks,
Ian