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 > What's a good battery level monitor?

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SJ-Chris

San Jose, Ca

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Posted: 11/18/20 01:18pm Link  |  Quote  |  Print  |  Notify Moderator

Regarding the drifts and possible inaccuracies due to a variety of behavioristics of charging and/or the batteries themselves....

I can see how IN THEORY you could have an issue if every time you charged your batteries you immediately STOPPED charging once your monitor said you are at 100%. If you were in fact not 100% full (...let's say the true SOC is only 95% even though your monitor incorrectly says 100%) and then you used your battery down to 50% as stated on your monitor, you would in actuality only be at 45% SOC. Then if you charged back up to what you thought was 100% and immediately stopped charging once your battery monitor says you are at 100% but in fact if it once again fell short you might only be at 90% true SOC. If you repeated this many times in a row, then I can see you'd have a problem. (drift)

Perhaps a good rule of thumb would be to make it a habit to continue to charge your batteries for a little while beyond the time when your monitor says you just reached 100%. Or, if your monitor shows amps going into your battery while charging, you can get a sense that way as to how much "charging" is still happening once your monitor reaches 100%.

This "drift" problem gets fixed/reset anytime you do have a situation where your charger stays on/active for a long enough time after your battery monitor says you are at 100%. For example, if you do find yourself plugged in at a campsite and your monitor reaches 100% (even if your true SOC is less then 100% because it has been drifting) while you are sleeping and then continues getting you to true 100% until you unplug. Or, if you are driving from one destination to another and your alternator is charging the battery...if your monitor gets to 100% (when your batteries are actually at less than 100%) the batteries will continue to get closer to 100% as long as you are driving. This "erases" any of the drift delta that might have built up.

Also: Most battery monitors tell you the Voltage it sees on the batteries. From most 12v SOC charts you can see that 12.06v resting voltage corresponds to 50% SOC. So if you ever see your battery resting voltage at 12.06v REGARDLESS OF WHAT YOUR BATTERY MONITOR SAYS percentage-wise, you should charge your batteries. Likewise, when your batteries are truly fully charged to 100% they will likely read ~12.7v at rest. If your monitor says it is 100% full but your batteries are only at 12.4v or 12.5v then you should continue charging until you get all the way up to 12.6-12.7v at rest. If your batteries never get there, then there is likely a problem with your monitor voltage reading (you can test by putting a voltmeter directly on the batteries) or a problem with your batteries.

-Chris


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Own two 2015 Thor Majestic 28a Class C RVs

BFL13

Victoria, BC

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Posted: 11/18/20 01:48pm Link  |  Quote  |  Print  |  Notify Moderator

Alternator charging should also pass through the House batts' monitor's shunt. The usual wire from the neg post to the frame for "grounding" is taken from the neg post and now goes on the shunt with all the other neg wires. Then there is one fat wire from shunt to neg post.

If you keep charging past when the monitor says 100%, then your AH counter will be out of whack and needs to be reset to zero once you do get to True Full.

The Trimetric default is to reset to zero when charging stops. It has no real idea what the true SOC is then. I don't like that whole business of how the Tri is supposed to work and the definition of "full" it uses. I do love my Tri though! I just use it "my way" [emoticon]

If you have solar and a Tri, it shows "charging" till it gets dark and then amps go negative again so it acts like you are full and resets. So they say to disable auto reset, which works great. Now it keeps counting up and down until sometime you manage to get the batts to True Full and you reset the AH counter to zero yourself.

I have read the Victron manual for how it works, but can't understand it. I would have to have one and see what goes on. Anyway, whatever monitor you have, you must stay on top of it and not let it mislead you.


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FWC

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Posted: 11/18/20 03:07pm Link  |  Quote  |  Print  |  Notify Moderator

It is not just theory. For many of us who don't plug in, you can get several partial charge cycles in a row (several days of non optimal solar for example) and as a result your SOC can be significantly off. Without plug ins, we also generally don't sit at float for hours upon hours to get your monitor back to 100%.

Yes you can make it work without these conveniences, but you have to remember when the last time you were on float for multiple hours, or try and check the battery voltage when you know things are at rest.

My point was that the good battery monitors deal with this, and you can be fairly sure when you glance at the display that the number is pretty close to reality. Cheaper monitors are a compromise.

I use lithium batteries, so the voltage doesn't tell me much about the SOC. However I have a good battery monitor that is correctly programmed for my batteries, and I can trust it.

SJ-Chris wrote:

Regarding the drifts and possible inaccuracies due to a variety of behavioristics of charging and/or the batteries themselves....

I can see how IN THEORY you could have an issue if every time you charged your batteries you immediately STOPPED charging once your monitor said you are at 100%. If you were in fact not 100% full (...let's say the true SOC is only 95% even though your monitor incorrectly says 100%) and then you used your battery down to 50% as stated on your monitor, you would in actuality only be at 45% SOC. Then if you charged back up to what you thought was 100% and immediately stopped charging once your battery monitor says you are at 100% but in fact if it once again fell short you might only be at 90% true SOC. If you repeated this many times in a row, then I can see you'd have a problem. (drift)

Perhaps a good rule of thumb would be to make it a habit to continue to charge your batteries for a little while beyond the time when your monitor says you just reached 100%. Or, if your monitor shows amps going into your battery while charging, you can get a sense that way as to how much "charging" is still happening once your monitor reaches 100%.

This "drift" problem gets fixed/reset anytime you do have a situation where your charger stays on/active for a long enough time after your battery monitor says you are at 100%. For example, if you do find yourself plugged in at a campsite and your monitor reaches 100% (even if your true SOC is less then 100% because it has been drifting) while you are sleeping and then continues getting you to true 100% until you unplug. Or, if you are driving from one destination to another and your alternator is charging the battery...if your monitor gets to 100% (when your batteries are actually at less than 100%) the batteries will continue to get closer to 100% as long as you are driving. This "erases" any of the drift delta that might have built up.

Also: Most battery monitors tell you the Voltage it sees on the batteries. From most 12v SOC charts you can see that 12.06v resting voltage corresponds to 50% SOC. So if you ever see your battery resting voltage at 12.06v REGARDLESS OF WHAT YOUR BATTERY MONITOR SAYS percentage-wise, you should charge your batteries. Likewise, when your batteries are truly fully charged to 100% they will likely read ~12.7v at rest. If your monitor says it is 100% full but your batteries are only at 12.4v or 12.5v then you should continue charging until you get all the way up to 12.6-12.7v at rest. If your batteries never get there, then there is likely a problem with your monitor voltage reading (you can test by putting a voltmeter directly on the batteries) or a problem with your batteries.

-Chris


SJ-Chris

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Posted: 11/18/20 03:21pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:


If you keep charging past when the monitor says 100%, then your AH counter will be out of whack and needs to be reset to zero once you do get to True Full.


I don't know how your battery monitor works, but I can share with you how I believe my Aili battery monitor works...

On the Aili monitor, you set the total AH of your battery bank and you also tell the monitor when you are charged to 100% initially to set it up. As you use your battery, your AH and % full value starts declining based on how much is draining out of your battery. Then when you are charging your batteries, the battery monitor will climb it's way up to 100% status and 100% AH as current is flowing into the battery. Once your monitor gets to "100% full" your AH will also be listed as 100% of your total battery bank AH. It cannot go any higher.

Now let's assume that in actuality you are NOT at 100% SOC even though your battery monitor says that it is. With the Aili, if you continue charging (either through solar or generator or plugged in or your alternator), your battery monitor will still just say 100% full (and AH at your max) as it continues increasing the true SOC until eventually you actually are at true 100% SOC. There is no resetting necessary on the AHs or the total capacity %.

Another way of thinking about all of this.... Let's assume that your batteries are at 50% SOC. 50% full. And let's assume you have 200AH total in your battery bank. Let's say you program your Aili battery monitor and you tell it that your battery bank is 200AH and your are currently at 100% fully charged (even though you are in actuality only at 50% SOC). If your next step was to plug in your rig for 10 hours charging the batteries (or driving for several hours while your alternator charges your batteries) your battery monitor would say "100% full" and "200AH" during the entire time. Eventually, your batteries WOULD BE at 100% and your monitor would now be showing you the proper SOC. No need to reset/etc.

And as mentioned before, at any time you can use the monitor to display the battery voltage to tell you where things are at.

Hope that makes sense.
-Chris

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Posted: 11/18/20 03:40pm Link  |  Quote  |  Print  |  Notify Moderator

This only works if you sit at float voltage for long periods of time. The current into the battery at float is typically only 1-2% of capacity, so if the monitor is off by 10%, it would take 5-10 hours at float to catchup. This may work for you, but not for folks that don't plug in every night, like the OP.

MrWizard

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Posted: 11/18/20 03:42pm Link  |  Quote  |  Print  |  Notify Moderator

Quote:

If you keep charging past when the monitor says 100%, then your AH counter will be out of whack and needs to be reset to zero once you do get to True Full.


NO !
My monitor and most other ones stop adding amps to the capacity value when the amps returned equal what was removed and set value is reached
Charging does Not Stop, but count does not increase beyond the set value
I think you are thinking of your way of a minus value from zero for amp hrs used then recharging to a plus value larger than zero to compensate and ensure you are at or near full,
Confusing what your display shows, with what is the normal operation of my system

* This post was edited 11/18/20 03:53pm by MrWizard *


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BFL13

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Posted: 11/18/20 04:13pm Link  |  Quote  |  Print  |  Notify Moderator

I get it now, I think! Does the ali show the amps still going in and tapering while the AH counter is frozen at zero when it thinks you are at 100%? When it gets dark and amps go negative, does the 100 start to go down then too? (kind of an auto reset?)

What about charging efficiency? The Trimetric has default setting for that (which you can adjust) ISTR 94% ? Anyway, that means it records the correct amps but adjusts the AH by that amount when charging-but not when discharging-, so that if the charging efficiency (allowance for heat loss) happens to be correct, AH back in will equal AH out. Since that allowance for heat loss will be off by whatever amount, your counter will "drift" and need to be reset.

The capacity entry that is "full" needs to be corrected for temperature and battery condition, so the %SOC will be wrong just from that. An AGM loses 15% capacity at 32F compared with its rating at 77F, eg. Do you re-enter your capacity according to average ambients when camping? Battery age and usage matter too. Say your battery loses 5% a year and you have 3 year old batteries (down 15%) at 35F (down 15% from that.) So what is your capacity for 100% that your monitor is using?

I prefer to have the actual approx capacity at the time worked out and figure out the SOC myself from the AH count. I just can't see how the monitor showing a percentage SOC can be believed, so I don't even have that displayed.

That is also where voltage /SOC is misleading--- the capacity can be down but the voltage per SOC is still close to valid. You can be at near full voltage with a smaller capacity. So the AH counter is valuable as a cross-reference to see if they "match" for SOC.

"Percentage of what?" has to be asked.

* This post was edited 11/18/20 04:21pm by BFL13 *

punomatic

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Posted: 11/18/20 06:02pm Link  |  Quote  |  Print  |  Notify Moderator

I have this one, and it does everything I need. I installed it with a DPDT switch so I can switch between reading during charging and during discharging.


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MrWizard

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Posted: 11/19/20 12:58am Link  |  Quote  |  Print  |  Notify Moderator

I full-time my RV, I use the LP furnace, basement is heated, there is no need to change battery capacity numbers unless the batteries are degrading

[image]
[image]Click For Full-Size Image.

Batteries fully charged
No need for me to worry about accuracy drift
I've said it here many times, I dont do 50>90 cycling of my batteries

BFL13

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Posted: 11/19/20 07:01am Link  |  Quote  |  Print  |  Notify Moderator

When I compare my "morning voltage" (close to resting) and its SOC with the AH count, that gives me a rough capacity. Since 12.1v is about 50% of whatever the capacity is, then if I am down 200AH, capacity is about 400AH.

I do a little math using the temperature vs capacity graph from my battery specs, and see if that is a reasonable number. There will be some more for battery age/usage.

My 460AH (as rated) might be 400AH on a cold morning, down 13% from rated.

If I did it the other way , using 460 as my capacity entry, and was down 200AH, the monitor would say I was at 57%. (the morning voltage would be 12.1v though-indicating 50%.)

By the time the monitor said 50%, it would actually be (230 down from 400) 42.5%.

On the recharge from actual 42.5%, I am unclear what the Ali would say. There is the business of whether battery temp is ambient or internal as it warms up from being recharged, and what temp to use with the temp vs capacity graph.

If the capacity is still lower, then the amps will taper to zero when it has its 200AH back (with heat loss allowance correct) before the AH counter gets its 230AH back. If capacity is closer to 460 again from internal heating, amps will run longer so the Ali comes out closer?

I do 50-90s with gen/charger so never get back up that high, and also get "progressive capacity loss" from repeated 50-90s. I can keep track of that doing my own math, but I don't see how a monitor could keep track of it all with a fixed capacity set for it.

In the summer with not so much temperature swinging (except in a desert) and closer to 77F, on solar getting to full most days, the monitor would not drift as much and also you have more chances to "zero" the AH counter too by getting to full so often.

I do find it confusing trying to imagine how different monitors would handle things.

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