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 > Setting Amp Hours In Battery Monitor

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1L243

Oregon

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Posted: 01/03/22 11:57pm Link  |  Quote  |  Print  |  Notify Moderator

I have a shunted battery monitor. It gives you alot of information. In setup one of the parameters is to input the amp hours of your battery bank.

I have four deep cycle lead acid batteries with a total of 340 Amp hours.

We know that lead acid batteries should not be taken below 50% or better yet 60% of charge.

As I use the power from the battery bank the monitor will show bank level in percentage of charge and available amp hours left so, at 60% I know I'm getting to a area where I have to think of getting amps into the batteries or shutting things down, even though it shows I may have 240 available amp hours left on the monitor.

Should I input the total 340 amp hours into the parameters settings of the monitor and just figure out as I have been doing or should I input the actual useable amp hours like 170 amp hours?


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SJ-Chris

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Posted: 01/04/22 01:17am Link  |  Quote  |  Print  |  Notify Moderator

Assuming your battery monitor STOPS going down (percentage-wise) when it gets to 0% on the monitor (regardless of what the true state of charge is) and STOPS going up when it hits 100%, you MUST program your controller with the full amount of AH of your battery bank.

Reason: If you only put 170AH as your battery bank (when you really have 340AH) you could easily have a problem knowing your actual state of charge. Consider the following situation.... Let's say you program your battery monitor telling it your battery bank is 170AH. Now lets assume you start with fully charged batteries and you start using/draining your battery while camping. Let's assume you actually use 340AH worth of battery power over a couple of days. As soon as you use 170AH your monitor will say "0% full" on your battery monitor. BUT, let's say you don't notice and you actually use 170AH more. Your battery monitor will still just say "0% full". You will incorrectly think that means your batteries are "actually at 50% state of charge". But in reality they are closer to 0% true state of charge (almost dead). Now assume you start charging and you charge your batteries 170AH worth of charge. Your batteries will ONLY be half charged (50% state of charge), but your battery monitor will now say they are fully charged up to 100% (when in fact they are not). Understand?

So program your monitor to 340AH and simply realize that when it is at 170AH (50%) you should think about getting them charged.

Note: It's been debated for years, but (my opinion) if ONCE IN A WHILE your batteries discharge 60, 70, 80% they will be just fine as long as you don't let them sit there for days/weeks/months at a time. Perhaps you will only get 400 "cycles" of your batteries instead of 500 (...or choose whatever numbers you want...). But ask yourself this: Will you be dry camping enough so that you will EVER cycle your batteries 300, 400, 500 times? For most casual RV users, they will never cycle their batteries this many times.

Hope that all helps!
Chris

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Posted: 01/04/22 09:20am Link  |  Quote  |  Print  |  Notify Moderator

Use the full Ah rating. 340Ah suggest Marine batteries which are compromised starting batteries. I'd suggest a 60% SOC limit. In the future consider GC2s which will provide 440 Ah, but they are 1" taller.

I like to say a Marine battery is not a deep cycle battery.


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BFL13

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Posted: 01/04/22 10:30am Link  |  Quote  |  Print  |  Notify Moderator

Yes, use the full capacity of the bank in AH so the AH counter will be able to monitor wrt to "full".

However you should not believe the percentage SOC by itself. Your entry of 340AH is for batteries in good shape at 25C/77F and batteries lose about 15% of their rated capacity when outside in the cold just above freezing, eg. That means there is a wide range of "full" but the AH counter does not know what the temperature is.

Also there is the problem of when you recharge the batteries to what you think is "full" you can be at less than truly full, depending on how that goes. That means your AH counter will be out of whack and get more out of whack with each recharge. This will make your "percentage SOC" a joke.

What to do? You look at your "morning voltage" before solar kicks in and jacks up the voltage, and when the furnace is not on so you get the nearest you can be while camping to "resting"--ie no loaded voltage making the voltage lower than it really is. Despite purists saying this is false info--it is "close enough" for the purpose of using the "morning voltage" to get the SOC from the voltage/SOC table for your type of battery.

Now you can compare that idea of SOC with your AH counter's idea so you will see if the AH counter is out of whack.

All that is true for batteries in good shape at their rated capacities. But what if yours are older and not in such good shape? 340 is too high. What is the correct number? You can tell from your "morning voltage" fairly closely.

Say your morning voltage is 12.1 and that means you are at 50% of whatever the real full capacity is. Your AH counter says you are down 150AH from when you last recharged to full and at that time you confirmed it was "true full" by hydrometer SG. (no good if you did not get to true full)

So you can say now that full is twice that 150AH --300 instead of 340.

If it is 35F out you can say -if the batts are still as rated- then the 340 should be about 15% less or 340-51 = 289. If the AH counter actually says you are down 125 at 50% by voltage, then full is 250 and it should be 289 so you know your batteries are not as rated anymore by about 39AH and are at 301 these days at 25C/77F (ignoring taking the 15% off 340 instead of 289 which number you didn't know yet--you can tidy that up if desired)

So now you could enter 301 into the monitor as your "full" and be closer to the truth. It will still get out of whack and you need to do the cross-check with the morning voltage and temperature factor. The percentage SOC readout should be treated as likely bogus at all times.

You are not done yet! The problem of not recharging to "true full" means you need to reset the AH counter to zero (called syncing with some monitors) whenever you do manage to get to True Full.

So monitors are a good thing, but not to be trusted unless often verified, and you pay attention. This can be against the grain for many RVers who are out there camping to relax and not have to think about stuff.

Too bad so sad, but if they just keep looking at their "percentage SOC" reading like it meant something, they can have their camping experience go bad with surprise dead batteries at 3am--which is not so relaxing.

In light of all that, the OP IIRC, said last year or so his four 12s were not in good shape, especially one of them, and he was going to replace them. If that "340" bank is the same bunch of batts, then goodness knows what the real capacity is to enter by now.


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CA Traveler

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Posted: 01/04/22 10:49am Link  |  Quote  |  Print  |  Notify Moderator

Battery monitors are an excellent guide and one learns by experience what the reality is. Sure if the battery is discharged at 3am (I only did that once) just adjust your reality for usage and charging for the following days.

BFL13

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Posted: 01/04/22 11:07am Link  |  Quote  |  Print  |  Notify Moderator

I guess we all did that once! Steep learning curve from there [emoticon]

pianotuna

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Posted: 01/04/22 11:39am Link  |  Quote  |  Print  |  Notify Moderator

1L243 wrote:

I have a shunted battery monitor. It gives you alot of information. In setup one of the parameters is to input the amp hours of your battery bank.

Should I input the total 340 amp hours into the parameters settings of the monitor and just figure out as I have been doing or should I input the actual useable amp hours like 170 amp hours?


It depends on if the meter uses a peukert calculation or not. If it does then use 340 OR do a full capacity test and use that number.


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

KD4UPL

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Posted: 01/04/22 05:33pm Link  |  Quote  |  Print  |  Notify Moderator

Use the full 340 number if that' what it is. To do otherwise will royally screw up the math in the monitoring device under certain conditions.

Are you sure it's 340 AH? That would mean you have 4 85 AH batteries in parallel I suppose?

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Posted: 01/04/22 09:06pm Link  |  Quote  |  Print  |  Notify Moderator

KD4UPL wrote:

Are you sure it's 340 AH? That would mean you have 4 85 AH batteries in parallel I suppose?
My guess as well.

1L243

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Posted: 01/08/22 09:45pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks for all the input. I will continue to set the monitor at 340 amp hours at 100%. The next time I go out I will pay better attention to percentage in relationship to amp hours. Last summer I seem to remember my percentage dropped mush faster than my amp hours.

I do have four 85ah deep cycle lead acid 12 volts for my battery bank.

I see that some quality used Lithium batteries are hitting the market with more than 90% capacity test.

Who knows maybe next year!

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