Battery Charging Complexities

[Peppercorn]

Hi, Can someone help me with understanding charging Car Batteries correctly?

For instance, I have a standard starting battery and another large standard small truck battery as my 2nd battery. Eventually, I will replace it with a Deep Cycle Battery!
I am using the 2nd battery to power my computer, sound system, and my Sleep Apnea Breathing Machine when camping. It will last about 2 days until it starts to show evidence of becoming flat.
The question is how long would I have to start my engine to recharge this battery fully??????? assuming the starter battery is fully charged?

I have been told over the years that from time to time a Car Battery has to be charged from a proper Battery Charger to get the battery charged up completely because an alternator cannot recharge a battery fully over time.
It is very confusing to me. So if anyone can help it would be much appreciated.
Thanks

[richard4u2]

i have been told a car alt will charge the battery to 75% with in 30 min then to fully charge could take hours , the best set up which i was told was run the car for 30 min then a sola pannel to top up the battery through out the day

[fraser130]

Peppercorn, There are now several brands of DC to DC 3 stage chargers, some as high as 35 Amps.
These will charge a auxilliary battery exactly like a proper charger, and give you an idea of how charged it is. If I had a device that I really needed to be available I'd just buy one and then maybe look at solar later.
they vary in price from just over the $200 mark, up to I think close to $1000.
I'll be getting one before this summer, as I've already destroyed a $500 AGM battery with a normal dual battery system (batt was very low, weather was very hot, and I drove for 4 hours. the TD5's alternator supplied too much current and the battery overheated, that was that)

Fraser

[101RRS]

I've already destroyed a $500 AGM battery with a normal dual battery system (batt was very low, weather was very hot, and I drove for 4 hours. the TD5's alternator supplied too much current and the battery overheated, that was that)

Fraser

Another example of how AGM battery technology is not appropriate for use in motor vehicles.

Garry

[PhilipA]

Garry, I have had an AGM ( Absorbed Power 90AH in N70 size)for almost 6 years now fed by its own 85 amp alternator with no problems. It is in the second battery position in a V8 RRC 92.

I only have a relatively thin wire to it from the second alternator which I see in my laymans ignorance as a current limiter. It charges to about 90% from 11 volts in about 2 hours driving and has been perfect, although now its getting a bit weaker than its 90AH would suggest.

So I think its not true that they are not suitable for car use in all cases. If you had one up against the Turbo in a diesel disco , then its probably too hot and probably lead acids would also have a short life.

By the same token I have read over and over that deep cycle batteries are unsuitable as second batteries as they cannot take fast charge.

Nothing has the same energy vs size as an AGM outside of a Lithium and at present that is very immature tech.
regards Philip A

[CraigE]

It can take 4 plus hrs solid driving to charge a car battery up from nearly flat. Just starting and idling a car does not charge very well at all.

[fraser130]

Sorry Gary, I disagree, no good as a main battery, and no good in the engine bay, but with a proper charging system, they are a fantastic battery.
Long lasting, vibration proof, install in any position, no explosive gasses.

Fraser

[drivesafe]

Hi Peppercorn, you are looking at at least 5 hours idling time to bring the battery back to around 90-95% SoC.

Next, if you use you vehicle as your daily drive vehicle and then drive for at least 3 hours going to and returning from your camping location, there is no reason to ever have to use a battery charge, on either battery, because alternators will not only fully charge your batteries, it will do both at the same time.

If on the other hand, you only drive it periodically, then charging the batteries would be a good idea.

A tip, if you find you need more power to be able to camp in one spot longer, simply add an additional auxiliary battery and you will take no longer to fully charge three batteries while drive than it takes to charge two. This is the biggest advantage of using an alternator as the primary charging source.

[101RRS]

Sorry Gary, I disagree, no good as a main battery, and no good in the engine bay, but with a proper charging system, they are a fantastic battery.
Long lasting, vibration proof, install in any position, no explosive gasses.

Fraser

Ok - but the scenario you described of the car alternator killing AGMs is coming up more and more in forums. They just do not seem to be able to take the variability of the in car environment. Having said that, there are clearly plenty of cases where the batteries work fine - but increasingly people are reporting issues.

Garry

[fraser130]

Ok - but the scenario you described of the car alternator killing AGMs is coming up more and more in forums. They just do not seem to be able to take the variability of the in car environment. Having said that, there are clearly plenty of cases where the batteries work fine - but increasingly people are reporting issues.

Garry

I wouldn't be surprised, I wasn't fully aware that they do need a bit more care than a wet cell (good thing I got 10 of them!), I will be getting a proper dc-dc 3 stage charger for next summer for the car.
I gather the main issue is they have such a low internal resistance that they can pull heaps of current from a modern alternator (mine pumps out 14.5v for a good while on startup and is rated at 110A), too much current and they heat up, then they can get into a runaway situation where they just cook.

[drivesafe]

Hi fraser130, where have you got your AGM mounted?

[fraser130]

Drivesafe, the one I cooked was in my 130, under the rear seat, just above the muffler. we were driving home from a short stay near wangaratta, it was 38 degrees, and the batt was probably near to flat, when I stopped at a stop near seymour I couldn't touch the battery it was so hot!
I subsequently mounted a heat guard above the muffler, and that helped heaps, but now I have a Trayon camper, and the new battery is in that, so heat isn't so much a problem, but I am definately fitting a dc-dc inverter style charger soon.

Fraser

[pawl]

A tip, if you find you need more power to be able to camp in one spot longer, simply add an additional auxiliary battery and you will take no longer to fully charge three batteries while drive than it takes to charge two. This is the biggest advantage of using an alternator as the primary charging source.

Hi drivesafe, if I did this would I just connect the two Aux. batteries in parallel then?

[drivesafe]

Hi Fraser, once you move the battery to the camper, you won’t need a DC-DC device, the cable length will protect the battery(s).

[drivesafe]

Hi drivesafe, if I did this would I just connect the two Aux. batteries in parallel then?

Yes Pawl, and as long as your have 6B&S then you should have no problems charging the batteries while driving.

[fraser130]

Hi Fraser, once you move the battery to the camper, you won’t need a DC-DC device, the cable length will protect the battery(s).

Possibly, but with it, I can't go wrong, and I can be sure I'm getting maximim charge, with current and voltage limiting.
I have 16mm2 DC welding cable all the way to the camper batt so over current could be a problem if the camper battery did end up too flat.
plus I know I'll get a full 3 stage charge profile even at an idle if I ever need it.

Cheers,
Fraser

[drivesafe]

Hi Fraser, before you waste your money on a DC-DC device, carryout some tests on your battery in the CT.

The reason I asked about the location of your battery is because I have never heard of a battery, AGM or otherwise, being over charged when it is fitted somewhere other than the engine bay, such as in the rear cargo area of a 4x4 or in a CT or caravan.

I have customers who have run 25mm2 cable to their caravan and never cooked a battery.

These DC-DC step ups are a bit of a con job.

Will they fully charge a battery, yes and will they do it quicker and better than an alternator, rarely.

If your battery only needs topping off than a DC-DC device will fully charge the battery quicker than an alternator can.

The problem with charging batteries while driving is that the lower the batteries are, when you start your drive, the longer you have to drive to fully charge the battery, no matter what type of charging you are using.

In most cases people NEVER drive long enough to fully charge their battery.

The biggest single drawback with having a DC-DC device is when you find you need more power than a single battery can supply so you add a second battery.

With your DC-DC device, you now need to drive about 90% longer to fully charge the batteries while just using your alternator, at very most you will only need about 20% more driving time.

For the very few occasions where these devices MIGHT do a better job than an alternator, the difference is so small that the cost of the devices is just not warranted.