battery charging

if I have a Century 46T (wet cell 50a/h deep cycle) under my bonnet and an AGM 100a/h under my seat in the car connected together will the standard charging from the Alternator work find on the 2 different types?

I currently have the 46T under my bonnet as a second battery. 50a/h is not enough but I can't fit a bigger battery. Looking at getting an AGM put under the passenger seat (the Remco is a long skinny one, not the standard 'box' shape) to give me 150a/h.

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Reply By: Chris11726 - Monday, Apr 19, 2010 at 18:23

Monday, Apr 19, 2010 at 18:23
Do you have a redarc or similar setup to charge the agm.

As long as you have this it will be fine.
AnswerID: 413566

Follow Up By: Gossy - Monday, Apr 19, 2010 at 20:07

Monday, Apr 19, 2010 at 20:07
Yes I have a redarc install which feeds into the second battery.

Because this will feed a constant charge into the batteries it doesn't matter if two different types are connected?
FollowupID: 683680

Reply By: CJ - Monday, Apr 19, 2010 at 20:11

Monday, Apr 19, 2010 at 20:11
The different types of battery do matter, and the redarc has nothing to do with it

Yes I agree get a redarc but for the other reasons.

Simply put, the two batteries, when connected, are in parralele and charges in parralel., Best then for the two batteries to be similar, or at least of similar size/capacity

But it will work, like many systems not quite 100% but hay, it is still better than one.

AnswerID: 413579

Follow Up By: Chris11726 - Monday, Apr 19, 2010 at 20:58

Monday, Apr 19, 2010 at 20:58
I can't see how it would matter. Yes they have different charging characteristics but.

Assuming the AGM is at 50%. You start the car, once the crank battery reaches 13.2v?? The red arc will put them in parallel and the AGM will begin to charge. AGM's have very little internal resistance and will take all the charge they can get which is why you need decent sized cables. Once you stop the car the red arc will keep the batteries in parallel until the crank battery drops down to 12.5v? then it will effectively disconnect the second battery.

Perhaps one of the battery guru's can shed some more light on the topic.

Either way I run a 100AH AGM second battery and just the stock lead acid as a crank and haven't had any problems so far.
FollowupID: 683689

Follow Up By: CJ - Monday, Apr 19, 2010 at 21:57

Monday, Apr 19, 2010 at 21:57
Yep, you are correct in all you are stating, There is no PROBLEMS you are right,
but it in NOT IDEAL

All I am adding is that it is ideal to have two similar sized batteries. This is one constant that all the gurus told me when I installed my second battery

But then again it looks as if you are bent on your setup anyway so why ask the question in the first place if you simply run the reply back to your original question.
As I said many setups on the forum, including my own is not ideal, but it is cheap and if 90% works for you great.

FollowupID: 683695

Reply By: trainslux - Tuesday, Apr 20, 2010 at 08:25

Tuesday, Apr 20, 2010 at 08:25
Yes, they will charge fine together.
Make sure you have good cables to the agm, and when you finish your trip/ use, top up the batteries with a good battery charger again.

Great idea to have more amp hrs with another battery.

AnswerID: 413609

Reply By: Gossy - Tuesday, Apr 20, 2010 at 08:49

Tuesday, Apr 20, 2010 at 08:49
sounds like it's going to work then :)

Will go for the Remco 105a/h which fits under the seats quite nicely. Agree it's not perfect but I can't get an AGM to fit under the bonnet and it's too hot in there for that type anyway.
this will give me a 150a/h solution rather than my current 50 which just isn't enough unless you are driving every day.

thanks for everyones advice.
AnswerID: 413612

Reply By: ChipPunk - Saturday, May 01, 2010 at 07:09

Saturday, May 01, 2010 at 07:09
Maybe a bit late on this (with apologies), but I've been dealing with these issues a lot recently and found some good sources....
One clarified the difference between needing matched batteries when paralleling full time (which is not recommended anyhow) as opposed to during charging.

The most common implementation seems to be a standard vehicle battery system with added "auxiliary" battery(s) for PCs, audio, fridges, etc.
These require an isolator to avoid the "not in use" parallel issue as well as provide independence to ensure the cranking ability of the original battery (engine etc).

BUT, instead of costly isolators, a relay is used.
The relay's coil is simply connected from earth/chassis to the alternator's or voltage regulator's charge lamp circuit.
The relay is normally de-energised, but when charging begins thereby switching the charge-lamp's earth to +12V and extinguishing it, the relay energises and connects the aux battery +12V to the main system.

Hence when the vehicle is charging, the batteries are connected in parallel.
This is NOT a problem whilst they are charging - irrespective of capacities and type. (Other than must be lead-acid type - ie, 12V wet, flooded, sealed SLA, Gel-cel, AGM etc.)
The batteries charge at their own rates based on whatever the alternator can output which should up to 14.4V and usually at least 13.8V.
Rarely will batteries take the full output current of an alternator, but if they do, they are still charged at the maximum rate the alternator can handle which is the fastest recharge rate available (in terms of combined system charge).

There are some other issues like what limit relay current the alternator can supply (0.5A to 1A seems typical which is more than most relays) but this can be easily overcome.
And the inter-battery cabling and fusing at each end (2 fuses!), but this is standard anyhow for non-adjacent parallel batteries.

I may not be explaining or describing this well enough, but I could link the source(s) I got my info from if anyone is interested.

I've used the same system on my truck and it's been great.
I use circuit breakers instead of fuses for the battery interlink as well as a low-voltage cutout from the second battery. I too had fuses blow due to some typical transient and then flattened the second battery so my fridge failed.
Not that I've had a failed interlink since the self-resetting circuit beakers were fitted, but if I do, at least I won't wreck my aux battery!
Costs were approximately $10 for a 60A relay, 2x$7 for 2x50A circuit breakers, and $20 for the low-voltage cutout, plus cabling, Anderson connectors etc.
AnswerID: 414933

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