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Fluctuating battery voltage

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Sam Longley 125/10/2018 22:24:38
698 forum posts
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Hi, I have a query re 12 V batteries on my boat & some of the caravaners/motorhomers on the forum may be able to comment as they also have domestic battery banks

I have a pair of 12 v 90A AGM batteries linked together as my domestic bank. I have a Dolphin 15 A triple stage charger that charges both starter & domestic batteries. It runs off shore power. If it is giving out max charge the cooling fan inside it goes on. If the fan does not go on I know that the batteries are not particularly low.
I have a NASA BM1 battery volt meter that works on the domestic bank only.
2 days ago I decided to check all was charged up for the laying up period, so I plugged into shore power. All electrics were turned off. The BM1 recorded 6 amp charge for a while at 13.7 V ( fan did not go on so I assume that batteries were fairly well charged)
I left it overnight & on return charge rate was 1 amp. Voltage still recorded 13.7 V
I disconnected the shore power (still showed 13.7V) & to empty the fresh water I turned on the tap & hence the fresh water pump. This then showed 1.3 amp discharge & voltage fell to 12.7 V within the 4-5 minutes I ran it. (I did not re connect shore power)
After about 15 minutes voltage went back up to 12.9 volts
The following day voltage was recorded at 13.1 volts
What I would ask is why such a sudden big drop- Should one expect this because 1.3 amps for 4-5 mins is really quite small ie. 0.1 amp hrs? Then why did it slowly go back up?
What is happening? No other electrics connected.No solar cells etc Is this a sign of knackered batteries?
Thanks

Edited By Sam Longley 1 on 25/10/2018 22:27:26

John Haine25/10/2018 22:33:40
2420 forum posts
132 photos

I think battery voltage does depend on temperature. As it was cold overnight the batteries would cool down and reduce voltage. When you draw current the batteries get warm since the electrochemistry isn't perfectly efficient, voltage goes back up. Just a theory.

Simon Williams 325/10/2018 22:42:23
365 forum posts
64 photos

 

 

Good evening Sam,

I asked myself the same question - albeit for different reasons - a little while ago, as I was trying to calibrate a battery condition meter (volt meter) to give some meaningful indication of the state of charge of lorry batteries I use to run PA amplifiers for a local fete. While they are running the PA they have no charge connected to them as they are in the middle of a field, so I wanted to know what the relationship between terminal voltage and state of charge was.

It's not a simple question, and I'm sure others will chip in with chapter and verse. but I found this which seemed to make sense.

http://scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

I think if you follow the graphs on the pages labelled 67 and 68, which show terminal voltage against charge and discharge currents, you can relate the difference in terminal voltage for a given state of charge to the current into or out of the battery.

HTH Simon

Edited - apologies, I'd linked to the Google page not the specific file/webpage.  Hopefully fixed.

Edited By Simon Williams 3 on 25/10/2018 22:50:21

Andrew Johnston26/10/2018 08:20:04
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4498 forum posts
520 photos

The voltage of a lead acid battery will vary considerably with charge/discharge current and to a lesser, and more predictable, extent with temperature. Allowable charge/discharge currents also vary with temperature. Despite what the linked article says the offload voltage of a lead acid battery is a very good indicator ot SOC. But the battery needs to rest for a number of hours after charging before measurements are made. Reading too soon leads to an over-estimate of SOC.

Andrew

Russell Eberhardt26/10/2018 10:05:39
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2417 forum posts
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It sounds as if your batteries are fine Sam. However I would recommend that you keep the charger on and connected to shore power all winter. Your three state charger will go into "float mode" when the batteries are full and that is the best way to preserve them as it will automatically compensate for any self discharge.

One question; does your charger have separate outputs for the domestic and engine batteries? If not, and you have a separate charge splitter, some of those (the ones that consist of just two diodes) can cause a problem as the charger output voltage will depend on the voltage of the lowest battery.

Russell

Sam Longley 126/10/2018 10:27:57
698 forum posts
26 photos

Russel,

Unfortunately I cannot keep it plugged in full time, but I do visit the boat every few days, so it gets a couple of hours a week all winter.

Yes, the charger has 2 outputs. The starter battery is a red flash 35 amp battery. That is small for a 20Hp diesel but as the engine usually starts on the touch of the button it is OK. I also have a switch to link all 3 batteries in an emergency if needed.

I note the point about charger output & did think of this. When i noticed this voltage issue I flicked the switch to link all 3 batteries to see what happened. It made no difference to the charge rate or to the voltage reading so I turned the switch off again.I wondered if the increase in voltage was because the domestics were drawing some power off the starter battery. ie through a faulty voltage relay on the engine alternator charging side.

However, reading from Simon's link it seems that it is normal for voltage to rise at restive state. Something that I had not noticed before. At least not to such a large extent.

It was the large initial drop from 13.7 to 12.7 in less than 5 mins that had me worried & I still do not understand why so much

peak426/10/2018 10:50:53
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724 forum posts
65 photos

Not got time to go into long winded explanations now, and if I did , I'd probably get some of it wrong. wink

Have a look at The Battery University for lots of good info on many types of batteries.

Bill

not done it yet26/10/2018 11:05:49
2719 forum posts
11 photos

The load accross the battery is shared as a function in proportion to the load resistance and battery internal resistance. All it means is that the battery is being warmed up while delivering external power. Nothing unusual. The nominal voltage of a lead acid cell is around 2.1Volts at full charge.

The battery must be charged at a higher than the battery voltage (or no current would flow into the battery) and this is what you see while charging. The first amp, or so, of charging current will do little, if anything to increase the battery SOC. Too much of course, when the battery is fully charged will electrolyse the water in the olectrolyte.

Most automotive alternators charge to 14.4V, but your AGM batteries are a different type to the common FLA cell.

Neil Wyatt26/10/2018 11:10:05
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15707 forum posts
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My leisure battery had dropped to around 11-12Ah capacity (I did three measured discharges with my iMax B6, it timed out twice) after a year or two's fairly regular use (roughly once a week). I suspect a combination of cheap charger and a couple of over-discharges due to two charger failures.

My guess is it has sulphated up rather than actually failed.

Can anyone recommend an economically priced reconditioner that has a decent chance of getting it up near its original capacity?

Robert Atkinson 226/10/2018 12:32:29
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167 forum posts
13 photos

Hi Sam,
As Andrew says the voltage will vary with charge, discharge and resting. Temperature also has an effect. It sounds like your batteries are fine.
Do you know what voltage your charger switches to trickle at? The initial charging voltage at 6A of 13.7V seems a little low and is probably due to the length / sixe of the cables connecting charger to battery. I'd expect to see at least 14.2V at the battery before it switched to trickle mode. I assume you were not there when it switched.

Robert.

Robert Atkinson 226/10/2018 12:42:48
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167 forum posts
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Neil,

Rather than a conditioner, if you have a variable DC supply cable of at least 18V (higher is better) with current limit it will do a good job. discharge the batteries. Set the current limit on the supply to 1/20th of the Ah rating of the battery (or 80-90% of the power supply current rating if less) and turn the voltage up maximum. Switch on and connect to battery and leave connected for at least 48h. If voltage is below 14.4 V, set voltage to 13.8V and leave forr another 48h. A lot of battery chargers have high ripple and lead acid batteries do not like this. a good DC supply is much better for them.

Robert G8RPI.

Sam Longley 126/10/2018 18:32:41
698 forum posts
26 photos

Robert

If the batteries are a bit low, ie after a cruise where I have used electric but not charged much with the engine then the charger will initially charge at 14.4 volts & a higher amperage, then when the fan shuts off the voltage drops to 13.7 as it showed in this case. Suggesting the batteries were fairly full anyway & charger was on stage 2 of the cycle.

Wires were the size recommended in the instructions when I installed the charger ( forget the size). Length is approx 2ft 6 ins to domestic & 3 ft 3 ins to starter.

Neil Wyatt26/10/2018 23:43:59
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15707 forum posts
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Posted by Robert Atkinson 2 on 26/10/2018 12:42:48:

Neil,

Rather than a conditioner, if you have a variable DC supply cable of at least 18V (higher is better) with current limit it will do a good job. discharge the batteries. Set the current limit on the supply to 1/20th of the Ah rating of the battery (or 80-90% of the power supply current rating if less) and turn the voltage up maximum. Switch on and connect to battery and leave connected for at least 48h. If voltage is below 14.4 V, set voltage to 13.8V and leave forr another 48h. A lot of battery chargers have high ripple and lead acid batteries do not like this. a good DC supply is much better for them.

Robert G8RPI.

Thanks, I've got several options for doing that.

My mistake was relying on cheap charger, and which expired spectacularly then getting a Lidl one that turned out to have four dry joints in it. Now fixed and seems to switch to trickle at about 14.4V

peak427/10/2018 00:32:14
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724 forum posts
65 photos

There's also various designs and kits available for battery desulphators (or desulfators as much of the info is from across the pond).

Essentially in parallel to a trickle charger, you add a gizmo which pulse loads the battery with an RF pulse intended to be at a resonant frequency for the plates.

I've had mixed success with the one I built, either improving things, or killing the battery completely.

See HERE as an example article. And HERE is an ebay link; mine is similar to the kit from Thailand, but was UK sourced, though I can't remember where from.

Bill

Russell Eberhardt27/10/2018 16:45:14
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2417 forum posts
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Posted by peak4 on 27/10/2018 00:32:14:

There's also various designs and kits available for battery desulphators (or desulfators as much of the info is from across the pond).

Essentially in parallel to a trickle charger, you add a gizmo which pulse loads the battery with an RF pulse intended to be at a resonant frequency for the plates.

I've had mixed success with the one I built, either improving things, or killing the battery completely.

See HERE as an example article. And HERE is an ebay link; mine is similar to the kit from Thailand, but was UK sourced, though I can't remember where from.

Bill

I'm not surprised you had mixed results. Those things are a total waste of money. If your battery is suffering from soft sulphation Robert's suggestion is best. If it is suffering from hard sulphation just scrap it.

Russell

peak428/10/2018 00:20:17
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724 forum posts
65 photos

Posted by Russell Eberhardt on 27/10/2018 16:45:14:

I'm not surprised you had mixed results. Those things are a total waste of money. If your battery is suffering from soft sulphation Robert's suggestion is best. If it is suffering from hard sulphation just scrap it.

Russell

I'm aware of the controversy surrounding them, and indeed was before I thought I'd try one out. wink

I'll admit to not having read all 650+ comments on the thread, but there's some interesting discussion on one of the threads on the link I posted earlier.

Bill

Muzzer28/10/2018 09:47:04
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2904 forum posts
448 photos

Yes, these things are utter rubbish. As a former engineering and research director at an industrial battery charger company, I looked into quite a few of these. They operate at the barely legal end of the market (in terms of claims) but the bottom line is they are successful at parting money from punters but not much else.The most successful ones have managed to obtain some form of patent application to strengthen the illusion but that generally doesn't prove anything.

Murray

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