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Old 11-11-2019, 15:36   #1
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Battery voltage discharge level.

I have 4 crown CR235 golf cart batteries on my boat. Typically when you look at voltage to decide discharge level you look at the resting voltage with no load.

E.g. 12.2 would be about 50 percent discharged, but this is no loads and then allowed for the battery to recover.

I have a victron battery monitor that also counts amp hours consumed.

When I am running my autopilot it draws significant current and the voltage while under load can drop to 12.2 V fairly quickly (after a few hours of use), but this is under load and the battery monitor only says maybe 35 or 40 AH have gone out of the batteries. I believe I have much more capacity to use, but I don't want to over draw the batteries. Is there a voltage level I should be looking at under load to make sure that I don't too deeply discharge them?
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Old 11-11-2019, 16:23   #2
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Re: Battery voltage discharge level.

It depends on the load, the more the load, the deeper the voltage droop.
Iím sure you know that just as an example starting the engine can drop it to 10.5 or so when they are fully charged, just for a second though of course
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Old 11-11-2019, 17:32   #3
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Re: Battery voltage discharge level.

I don’t let mine drop below 11.5 under load while underway before doing something to charge them. Last week during a 3 day passage this was in middle night at about 60% soc.

Since I was going to have to recharge the battles no matter what I see reason to let them go lower just to have to run the engine ever longer. I only run the engine till acceptance amps start dropping.
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Old 11-11-2019, 17:53   #4
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Re: Battery voltage discharge level.

Voltage is an unreliable state of charge indicator, unless the batts have been rested for 10 hours or so. That's impractical.
For a capacity meter to work accurately, it must know the REAL capacity of the batts. NOT whats written on the side or in the PDS.
It sound to me that with the voltage dropping off that quickly, that the batteries may have reduced capacity. Take them out, and have them properly tested with a 20 hour discharge test. Then reprogram the meter with the real capacity.
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Old 11-11-2019, 17:56   #5
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Re: Battery voltage discharge level.

You have 4 GC batteries, most likely in the range of 400-450 ah. You're using 35-40 ah? No worries, mate!

Really, think about it, you're using 10% of your bank's capacity (40/400), 20% if you only go to 50%, right? 200 usable ah in a 400 ah house bank.

For the past few cruises I've done, I've kept hourly track on a spreadsheet form I made of my ah use and voltage. After a few two or three day trips anchored out, I started to forget about voltage, 'cuz it just doesn't work as any useful indicator. ah do.

With my fridge running 24/7 but of course at the 50% use for 60 ah per day (5A * 24 hours * 50% run time), my voltage pretty much gets to 12.3 or 12.2 pretty soon after engine shutdown.

I agree with sb1, once it gets down to 11-something it's time to put some amps back in.

That spreadsheet taught me a lot, and I've had this system in for 15 years! I have a 390 ah house bank of 3 Grp 31 wet cells. I have a Link 2000 BM.

In SF or 18 years with this boat I did more sailing than motoring, my old ST3000 autopilot doesn't draw much, but I never thought it a big part of my daily load, the fridge is. Here in BC I am a trawler with a stick and motor, so my bank is almost full when I pull in to anchor or raft up to a park float (dock).

You should be just fine.

Maine Sail keeps saying don't let your bank go below 12.2V, but mine goes there and stays there for more than a whole day.

You might try keeping track a few times just to convince yourself. I did.

Good luck.
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Old 12-11-2019, 01:50   #6
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Re: Battery voltage discharge level.

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Originally Posted by Stu Jackson View Post
You have 4 GC batteries, most likely in the range of 400-450 ah. You're using 35-40 ah? No worries, mate!

Really, think about it, you're using 10% of your bank's capacity (40/400), 20% if you only go to 50%, right? 200 usable ah in a 400 ah house bank.

For the past few cruises I've done, I've kept hourly track on a spreadsheet form I made of my ah use and voltage. After a few two or three day trips anchored out, I started to forget about voltage, 'cuz it just doesn't work as any useful indicator. ah do.

With my fridge running 24/7 but of course at the 50% use for 60 ah per day (5A * 24 hours * 50% run time), my voltage pretty much gets to 12.3 or 12.2 pretty soon after engine shutdown.

I agree with sb1, once it gets down to 11-something it's time to put some amps back in.

That spreadsheet taught me a lot, and I've had this system in for 15 years! I have a 390 ah house bank of 3 Grp 31 wet cells. I have a Link 2000 BM.

In SF or 18 years with this boat I did more sailing than motoring, my old ST3000 autopilot doesn't draw much, but I never thought it a big part of my daily load, the fridge is. Here in BC I am a trawler with a stick and motor, so my bank is almost full when I pull in to anchor or raft up to a park float (dock).

You should be just fine.

Maine Sail keeps saying don't let your bank go below 12.2V, but mine goes there and stays there for more than a whole day.

You might try keeping track a few times just to convince yourself. I did.

Good luck.
I did read Mainesail's article and I have an alarm set to when battery voltage gets to 12.2V. It went off after probably 5 or 6 hours of Autopilot use the other night (I just installed it), but the amp hours used from the batteries were quite low. For sure, the load was dragging them down, and I understand instantaneous voltage isn't a great indicator of actual state of charge. I'd need to turn everything off and see what they recover to for a few hours for it to actually be a worthwhile measurement. We typically use about 55-60 AH overnight which is mostly the refrigerator when we are anchored then the Solar Panels recharge them. Our victron battery monitor counts AH's in and out so you can see how much is used. I understand that total capacity declines over time with use, so the percentage of the state of charge isn't necessarily correct, but the AH count used should be. I have a 470AH bank (quoted capacity) that is less than a year old, so they should be in pretty good shape still.
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Old 12-11-2019, 04:24   #7
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Re: Battery voltage discharge level.

Seems to me that either you have lost a LOT or battery capacity or that the batteries weren’t fully charged and the info on the BM is wrong, or your AP is really drawing a lot of power.

Have you checked and cleaned your connections?

6 hours into a night sail I’m down 90 ah. I don’t know how in post 1 you can only be down 40ah.
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Old 12-11-2019, 07:30   #8
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Re: Battery voltage discharge level.

You need to get your adjustment and usage of the BM to the point where it is at least reasonably accurate.

Peukert, CEF and regularly adjusted SoH% lowered actual Ah capacity.

So you can then ignore voltage under load as an indicator.

Replace the bank when actual capacity is 75-80%, or when you notice its performance has dropped, e.g. voltage sag from loads much deeper and faster.
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Old 12-11-2019, 07:31   #9
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Re: Battery voltage discharge level.

Go here https://marinehowto.com And CTRL-F for battery monitor and read closely those articles.

Especially this one https://marinehowto.com/programming-a-battery-monitor/
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Old 12-11-2019, 11:44   #10
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Re: Battery voltage discharge level.

Following. I derate my 6 Trojan T-105s by about %/yr, ie, new=675 but I start with 650AH installed in my Victron 712. Yr2 = 615AH, yr3 = 590, yr4=560, yr 5=525, etc. I've never had the luxury of doing a 24hr 20amp load test, but I'm guessing that derating ea yr and using a conservative avail AHs, and then staying above an 80% SOC should help preserve their life?
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Old 12-11-2019, 14:33   #11
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Re: Battery voltage discharge level.

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Originally Posted by sailorboy1 View Post
Seems to me that either you have lost a LOT or battery capacity or that the batteries werenít fully charged and the info on the BM is wrong, or your AP is really drawing a lot of power.

Have you checked and cleaned your connections?

6 hours into a night sail Iím down 90 ah. I donít know how in post 1 you can only be down 40ah.
Or... his autopilot has something seriously wrong. If there's a solenoid in the mechanism, those can go bad and start drawing far more than they're supposed to. It's possible his autopilot is just draining the crap out of his bank. (though I agree the batteries are a better suspect)
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Old 12-11-2019, 15:59   #12
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Re: Battery voltage discharge level.

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Originally Posted by Jeremy-KHYC View Post
Or... his autopilot has something seriously wrong. If there's a solenoid in the mechanism, those can go bad and start drawing far more than they're supposed to. It's possible his autopilot is just draining the crap out of his bank. (though I agree the batteries are a better suspect)


Only problem with assuming the AP is drawing more than it should is that heís not down much in AH.
If it were drawing a lot, he would be down a lot of AH.

I suspect batteries too, but would start with cleaning the terminals real well and seeing if it makes any difference.
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Old 12-11-2019, 16:07   #13
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Re: Battery voltage discharge level.

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Only problem with assuming the AP is drawing more than it should is that heís not down much in AH.
If it were drawing a lot, he would be down a lot of AH.

I suspect batteries too, but would start with cleaning the terminals real well and seeing if it makes any difference.
Not sure... He says it's down 35-40 amp-hours after "only a few hours"... if that number is based on measuring actual amps drawn over time, that's a hard measurement and it says the autopilot is probably drawing too much (imho) (10+ amps constant draw from a solenoid or small motor? That's really high in my experience).

If on the other hand it is a calculated amp-hour loss from a voltage reading, that's more in-line with the batteries being a problem, or the calculation of their total capacity being wrong as has been said before.
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Old 12-11-2019, 17:55   #14
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Re: Battery voltage discharge level.

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Originally Posted by felizcortez View Post

I have a victron battery monitor that also counts amp hours consumed.
These do Ah-counting directly in order to guesstimate SoC.

So current voltage is displayed, but not the basis of the SoC estimate.

If you have Peukert & CEF tweaked just right, on the way up while charging, hitting the 100% mark will coincide pretty well with the (canonical) point endAmps is reached.

Having residual capacity overestimated just means your actual discharge cycles are going deeper than the supposed SoC% readout, ever accelerating the bank's approach to EoL.
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Old 12-11-2019, 18:18   #15
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Re: Battery voltage discharge level.

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Originally Posted by Jeremy-KHYC View Post
Not sure... He says it's down 35-40 amp-hours after "only a few hours"... if that number is based on measuring actual amps drawn over time, that's a hard measurement and it says the autopilot is probably drawing too much (imho) (10+ amps constant draw from a solenoid or small motor? That's really high in my experience).



If on the other hand it is a calculated amp-hour loss from a voltage reading, that's more in-line with the batteries being a problem, or the calculation of their total capacity being wrong as has been said before.


No, thatís the whole boat draw, refrigeration, plotter, wind instruments, depth finder, lights, radio, AIS everything. I find 10 amps to be not high at all for a boat under sail and autopilot.
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