Thread: Mixing AGM and wet cells...

We've been running a 92Ah AGM battery in the rear LHS of storage bin.
It is connected to the main battery via one of those smart relays that connect it once the main battery has reached a certain voltage. We got this from battery world. The wiring runs from the +ve main battery terminal via fuse in engine bay through the firewall just above the accelerator pedal (there's a large rubber grommet with a thick loom running through it which I used) then it runs up the driver's side A pillar, across the roof lining, down the C pillar and into the rear bin (at the time I figured it was easier this way than under the carpet). The relay is mounted inside the bin just above the battery.

The cable was that nice thick oxygen free copper high current stuff from Jaycar (4AWG WH-3064) and the relay kit had all the rest of the connectors required. Only ran the positive cable and using the vehicle chassis as the earth.

This all seems to work great except that the 2nd battery is now stuffed. Turns out it was stuffed before I put it in but I didn't know it. It works well enough to keep the fridge (Engel 40) running for a few hours during a picnic or something but not enough for overnight.

Anyway the point is the main cranking battery is fine. It gets enough charge even with the 2nd battery requiring more of a top up than usual. The relay disconnects them both when ignition is off so this also stops the 2nd battery from flattening the main one.

I'm not sure I'd put a flooded cell in the cabin. If it started to produce too much gas for what ever reason then you'd initially get a few fumes and eventually some serious rust neither of which can be real good for anything. I also had a cranking battery explode in the engine bay of my old XP a few yearsa back. It was not a faulty alternator and the battery was brand new but was caused by a spark that ignited some gas. I sounded like a back fire. I'd hate for that to happen inside or under my seat not to mention having to wash out the acid afterwards.

HTH cheers
Peter

Originally Posted by knp

I also had a cranking battery explode in the engine bay of my old XP a few yearsa back. It was not a faulty alternator and the battery was brand new but was caused by a spark that ignited some gas. I sounded like a back fire. I'd hate for that to happen inside or under my seat not to mention having to wash out the acid afterwards.

HTH cheers
Peter

To say nothing of having to wash out your pants

Originally Posted by slug_burner

To say nothing of having to wash out your pants

Yup!

I'm running a calcium wet cell under the bonnet as my starter battery and an AGM (for the fridge, etc.) in the rear underfloor wheel well. It is charged via an SC40.

I picked the AGM as the wheel well is reasonably well sealed (well, not leaky like a Defender!) and it also contains my LPG tank. The AGMs won't spill acid if they fall over or, I believe, if the case splits).

1. Gas is typically only produced in significant quantites if the battery is overcharged hence why the caps are taken off a normal Pb-acid battery when charging and why charging should occur in a well ventilated area. I agree the biggest danger with them is acid leakage from a roll over or excessive bumping. Normal alternator charging won't overcharge your battery.

2. Both normal car batteries and the deep cycle are Pb-acid but the deep cycle are sealed.

3. They can be charged together and run together. Your alternator won't charge the deep cycle to it's capacity as the alternator is typically 80-90% efficient. Good to give your deep cycle a top up with a balance charger every now and then.

4. The batteries are designed differently. Normal car battery designed to give a large current over a short period (hence thin plates more surface area). A deep cycle is designed to give a steady current over a longer time (thicker plates). That is why deep cycle are recommended for applications such as fridge and lights in camping situations where current draw is low but sustained for longer periods. Car batteries will work but won't like being discharged too much (say 60-80% might be ok) and recharged.

5. A deep cycle can be discharged over several hundred discharge/recharge cycles. A typical car battery is not designed to be totally discharged and if it does to many times the thin plates weld together or fail.

6. Deep cycle can be discharged almost completely but it isn't good for them. Typically if you want to maintain a long life then discharge them to around 30% and recharge them to 100% (i.e. special charger when you return from hols) to top up the alternator which only gets them to 80-90%

Hope that helps.

Hi Remy, the only reason an alternator doesn’t fully charge any automotive battery is because the alternator has not been given the chance too and not because it can’t.

As long as the alternator has an output voltage of 13.8 or higher, there is no reason why an alternator, given a reasonable time, can not fully charge any 12 volt battery.

This myth that states an alternator can only charge the vehicle’s battery(s) to around 70 to 80% is based on what I call “Shopping Trolley Syndrome”, where the vehicle is only ever driven for short periods of time and as such, the alternator is never given enough time to fully charge the cranking battery.

This is not the same as the alternator can not fully charge the battery, when in actual fact, the alternator in any vehicle can not only fully charge the battery, when there are two, three or even four batteries, an alternator can charge them quicker than any of the devices being pushed these days, with claims they are the only way to get your batteries fully charged.

This is no more than sales hype being fed to unsuspecting people who have no real idea of how batteries and alternator function.

Cheers.

Hi drivesafe,

Completely agree that the alternator needs to be given a chance to charge the battery (limiting discussion to deep cycle batteries) and that in many application it isn't (take for example a "typical" family camping trip with a long trip to start to get to the destination (battery semi-full http://www.aulro.com/afvb/images/smilies/smile.gif) followed by short trips once at the destination- battery not given enough charge to semi-fill). There are many other factors that will compromise the alternators job, all centered around load e.g. stereos, fridges, aircon, lights (spotties), number of batteries as you point out and not the least of which is temperature which effects the batteries ability to accept charge and charging rates. The last of which differs depending on the architecture and chemistry of the battery type.

Now the issue here for DEEP CYCLE BATTERIES isn't strictly the output of the alternator (but it is important - noting your comment 13.8v which i will get back to) it is more the voltage regulator (closed loop controller) which controls the output of the alternator and prevents it rising above a set level (e.g 13.8v your value but they range and after all we are talking LR alternators http://www.aulro.com/afvb/images/smilies/smile.gif). Once the battery reaches a higher charge (say 50-60%- they are different) the regulator begins limiting the charge going in until it reaches its threshold value (your 13.8v). In normal car batteries the regulator replaces relatively small voltage drops ( taken out by starting the car) fairly fast i.e. a low charge but does it fast.

In deep cycle batteries due to the architecture of the cells you need to overcome battery counter voltage which increases as charging increases (hence deep cycle batteries typically use a high voltage charge to overcome this say 14.4 volts for a short period 15-20min). This voltage is above what your standard car regulator allows 13.8v). Without this high charge the standard alternator regulator system cannot fully charge a deep cycle battery and typically will only charge it to ??? % for arguments sake 80.

Now to this "This is no more than sales hype being fed to unsuspecting people who have no real idea of how batteries and alternator function"

Deep cycle battery manufactures have spent a great deal of money developing specialised chargers/ regulators that cost a bomb to deal with these issues. I don't think it is just to fleece the public. The architecture and chemistry of these batteries differ from your standard Pb-acid car battery and therefore the charging systems have been adapted to suit. In application where deep cycle batteries are bieng used your standard automotive regulator can be replaced with a SMART regulator to overcome these problems.

I hope that clears up some of the myths.

Cheers
Remy

P.S. for normal car batteries your statements are correct.

Sorry Remy, I don’t know where you are getting your info from but you have it wrong.

It makes no difference what type of 12 volt battery you have, ALL 12 volt batteries can be fully charged AT ROOM TEMPERATURES with a supply voltage of 13.8 volts. Batteries under the bonnet will actually fully charge form a marginally lower voltage because of the increased temperature.

Next, most vehicles, including Land rovers, have an average output voltage of 14 to 14.2 volts and many of the Disco 2s have a working voltage of 14.7.

If your vehicles working voltage, the operating voltage after the vehicle warms up, is just 13.8 volts, it’s time to get the reg looked at.

Again, so there is no misunderstanding, all lead acid batteries can be fully charged with a supply voltage of 13.8 volts. It may take longer to charge some types of batteries compered to others but it is still the minimum voltage for any battery to be able to get a full charge.

Cheers.