Dual Battery Charging Behaviour

Thanks Tim. Agree with what you’ve said.
I’ll be putting a switch in the earth wire of my old SC80.
And will continue to regularly charge the batteries with the smart charger.

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Originally Posted by drivesafe

Even when these isolators are left in SHARED Mode, they only stay on for 72 hours and would consume roughly 12 amperes of energy during that period.

The problem being shown here ( and in other related threads ) is that batteries are dropping in voltage levels far quicker than if they had discharged just 12 amperes of capacity.

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Folks, you have to maintain your batteries to keep them in good condition, to reduce sulphration building up.

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Hi Tim,

Thanks for your information and knowledge sharing. I'm just trying to get my head around this.

Part 1 question: Your post indicates 12Ah of drain over the 72 hrs of shared mode due to the SC80. In my case I see the shared voltage drop to around 12.4V or just above before disconnecting. This would generally be considered around 80% SoC for AGM as I understand it. I have a combined 150Ah (C20) of battery (95Ah main + 55Ah aux) so that lost 20% equates to around 30Ah which is nearly three times the SC80 drain. In the three days after disconnection I'll see much less than an additional 0.05V drop on the main (and nothing on the aux) which equates to less than 5% SoC or <5Ah. To be honest the voltage drop after isolating is negligible and hard to measure accurately. To me those numbers just don't align so I am looking to understand if there is something I've missed. Is there some sort of interaction that keeps the car awake more while the SC80 is linked? Why does the voltage of both batteries recover nearly 0.1V as soon as they are unlinked? Am I better off bypassing the SC80 at the moment and hard-connecting both batteries while still in infrequent semi-lockdown use?

In my Disco 1a I have two Optima 55Ah blue-tops. Either side of the engine bay. One is pretty old, the other is less-old. Hard connected via a simple rotary isolator switch, but I leave them connected. I can (and regularly do) leave that vehicle for two months and hop in it and fire it no worries. That's a very high compression large(ish) capacity V8. Granted lower parasitic losses but also less battery capacity and a much, much longer timeframe. If I leave my L320 for just a week it'll grind over slowly and not start with confidence at all (TDV8) whilst reporting a very low cranking voltage.

Part 2 question: What's your recommended recovery/maintenance cycle? Is it something cyclical like solar (charge, rest, charge, rest, charge, rest, etc)? Is it hook up to a reputable 5-7 stage smart charger and just leave plugged in all the time? Should both batteries be connected when charging or individual?

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Originally Posted by drivesafe

Try disconnecting the thin earth wire on the SC80 and then see if the batteries discharge so quickly.

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Continuing my learning journey. Have disconnected the SC80 but have not been able to leave the car unused to observe the discharge pattern of the main battery when isolated. The isolated aux drops about 0.08V a day. Interesting how engine heat cycles seem to increase volts, despite being disconnected.

https://uploads.tapatalk-cdn.com/202...b862b2b0fb.jpg

I’ve always done a monthly or more frequently charge on the CTEK, and given the load test result, I think the batteries are ok, but I will get them shop tested to confirm sometime. The rate of discharge for the linked batteries is too high for the SC80 drain alone, and the isolated aux discharges too quickly for the ABG25 alone. So I think have a small parasitic drain somewhere, but not large enough to bother chasing just now.

I understand the DBS much better now with all this testing, so enough for now. Thanks all for your interest.

Hi again Winglen, try disconnecting the earth/negative ( - ) lead from your auxiliary battery and leave it that way for a few days and see if it still discharges.

You definitely seem to have fast discharging?

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Originally Posted by DiscoClax

Hi Tim,

Thanks for your information and knowledge sharing. I'm just trying to get my head around this.

Part 1 question: Your post indicates 12Ah of drain over the 72 hrs of shared mode due to the SC80. In my case I see the shared voltage drop to around 12.4V or just above before disconnecting. This would generally be considered around 80% SoC for AGM as I understand it. I have a combined 150Ah (C20) of battery (95Ah main + 55Ah aux) so that lost 20% equates to around 30Ah which is nearly three times the SC80 drain. In the three days after disconnection I'll see much less than an additional 0.05V drop on the main (and nothing on the aux) which equates to less than 5% SoC or <5Ah. To be honest the voltage drop after isolating is negligible and hard to measure accurately. To me those numbers just don't align so I am looking to understand if there is something I've missed. Is there some sort of interaction that keeps the car awake more while the SC80 is linked? Why does the voltage of both batteries recover nearly 0.1V as soon as they are unlinked? Am I better off bypassing the SC80 at the moment and hard-connecting both batteries while still in infrequent semi-lockdown use?

In my Disco 1a I have two Optima 55Ah blue-tops. Either side of the engine bay. One is pretty old, the other is less-old. Hard connected via a simple rotary isolator switch, but I leave them connected. I can (and regularly do) leave that vehicle for two months and hop in it and fire it no worries. That's a very high compression large(ish) capacity V8. Granted lower parasitic losses but also less battery capacity and a much, much longer timeframe. If I leave my L320 for just a week it'll grind over slowly and not start with confidence at all (TDV8) whilst reporting a very low cranking voltage.

Part 2 question: What's your recommended recovery/maintenance cycle? Is it something cyclical like solar (charge, rest, charge, rest, charge, rest, etc)? Is it hook up to a reputable 5-7 stage smart charger and just leave plugged in all the time? Should both batteries be connected when charging or individual?

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Hi DiscoClax, you have not missed anything, I have not explained it fully.

You can have a battery that is unused for a few weeks and you measure the voltage at the terminals and get say 12.5v.

But the instant you try to start your motor, the voltage drops and you may not even be able to get the motor to turn over, let alone start.

The voltage reading you got was an OPEN CIRCUIT voltage reading and while commonly used, IMO is next to totally useless.

An OPEN CIRCUIT voltage reading measures the State of Charge of the battery's capacity that is available, but does not give any indication of just how much capacity there is.

In the above example, there was actually very little capacity available, but that very small amount of capacity was near fully charged.

Whereas, with one of my isolators connected to your batteries, and running, there is a constant and more importantly a known load discharging your battery.

Any load applied to a battery will cause the battery's voltage to indicate a much more accurate state of charge based on the available capacity of the battery.

With fully charged batteries, in a good condition, my older SC80 isolators would take around a month discharge the batteries down to 12.0v, the shut-off voltage level of the SC80.

You can do some simple math and make up a graph of where you battery voltage should be and then compare with where it actually is.

As both yours and Winglet's batteries are discharging too quickly, and while your isolator is causing the discharge, as it is a known load, my "GUESS" is that your batteries are either old, or most likely, need some rejuvenating charging.

Quote:

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Originally Posted by drivesafe

Hi again Winglen, try disconnecting the earth/negative ( - ) lead from your auxiliary battery and leave it that way for a few days and see if it still discharges.

You definitely seem to have fast discharging?

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Hi Tim,

Have used lockdown for some more testing. As you suggested, I have disconnected the -ve aux terminal (and reconnected BM2 to monitor) and the SC80-12 earth and left the car double locked for four days after a week of reconditioning charging.

Result is the main dropping 0.02V/day and the aux remaining rock steady. In this configuration, I could leave the car unattended for over a month.

https://uploads.tapatalk-cdn.com/202...d5a2f6b5fb.jpg

This is a dramatic reduction in discharge rate compared to earlier with the batteries linked and SC80-12 active when they dropped 0.10V/day.

So I either have a drain on my aux circuits or the isolator is drawing more than expected? As I said in entry #11 of this thread, the only items on the aux circuits are a UHF (off) and the Traxide kit. I wonder whether it worth trying one of your upgraded isolators? Will PM you on cost and difficulty to install as replacement.

Cheers,

Chris