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Old 08-06-2019, 01:51 PM   #1
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Default What a battery monitor may not tell you

There have been a few battery monitor discussions lately, which I think is good as monitors are a very good thing for many of us.


We have had monitors for a long time, so we know exactly what to expect, but for new, or non technical, users that is not the case.


I just ran a test of what could be an easy to have use/charge cycle to watch how the monitor and charging reacted.


I ran the batteries down about 20%, did a short drive to put some charge back in ( could have been longer drive and lower batteries), then parked the van inside so no solar and plugged it in to shore power.


As expected, the shore charger didn't recognize that a charge cycle was needed because of the residual surface charge from the drive holding the voltage at above 12.8v. The monitor still showed about 10% down. The charger went to float and I let it run for 12 hours to test the question of if float will fill the batteries in the top off area of the charging curve. At that point the monitor had just shown 100% charge and net of zero in and out amps, so it would appear charged fully. We know that is not the case , though, as you always need to return 10-20% more amps than used. I then forced the charger into absorption voltage and after a couple of minutes the charging amps settled at about 10 amps, which would indicate we needed about another 3 hours of absorption charging in our system to get to the 2 amp fully charged point.


This answers a couple of the questions that have come up, I think, at least partially.


The first is if the typical charger that stops absorption early will get the batteries to full on float? The answer would be that it will not in an overnight charge situation which is very common for most of us.


The other question is if what you see on the monitor is a accurate and/or correct? I think that the answer to that is all of the above, unfortunately. The zero in/out amps is accurate, but it doesn't mean the batteries are full. The 100% SOC is a guess that the monitor made based on what it thought the charge efficiency would be. On this particular monitor, which does not light a separate "fully charged light", the only way to confirm full would be to force an absorption charge and look at the amps to the batteries.


The no fully charged light situation is on both our monitors, which are both built into the remotes of the shore charger and solar controller. My guess is that they don't have the lights because the shunt based setup for them is an option. For this reason, a standalone monitor with a fully charged indicator is a better choice for indicating really full charging, especially for new users or the non technical ones. On that kind of monitor, if the light isn't lit, the batteries aren't full yet, and that is very easy for everyone to use.
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Old 08-06-2019, 02:25 PM   #2
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Are you discussing lead acid batteries only?
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Old 08-06-2019, 03:23 PM   #3
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If I recall correctly you have Magnum inverter with ME-ARC50 and ME-BMK shunt. Did you test this combo?

I think any independent shunt battery monitor with interrupted charging by different sources will be prone to errors like for example residual surface charge. An appropriate load to remove surface charge before connecting new charge source could help.
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Old 08-06-2019, 03:47 PM   #4
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Here is a link to a very thorough discussion of using battery monitors in the context of marine batteries. It does not seem to address lithium, but it details the limitations for lead acid batteries and some that probably apply to both. It both describes amp-counting battery monitors limitations and gives specific instruction on how to better deal with those limitations.

I would read it before you invest in a battery monitor that will not give you the accurate information you are looking for. If you have a battery monitor, I would read it to get a better understanding of how to get the most out of it. I was particularly interested in his description of how improperly using one can lead to shorter battery life.
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Old 08-06-2019, 04:25 PM   #5
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Interesting info from Booster. Somewhat similar to what can happen with a basic PD converter when plugging in and there's sufficient surface charge to put it straight into normal 13.6V mode.

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Originally Posted by booster View Post
....................... For this reason, a standalone monitor with a fully charged indicator is a better choice for indicating really full charging, especially for new users or the non technical ones. On that kind of monitor, if the light isn't lit, the batteries aren't full yet, and that is very easy for everyone to use...............
An exception would be like how the Samlex EVO inverter charger works. My EVO goes into bulk mode whenever it is reconnected to shore power regardless of voltage. The batteries can be at 13.2V on solar float and the EVO still always goes through the full charge cycle when reconnected to shore power. That charge cycle; bulk, absorption then float can be very quick (like minutes) if the batteries are actually full (to my programmed criteria) because bulk to absorption is triggered by voltage and absorption to float is trigger by current flow. Note: It doesn't have an SOC monitor.

Basically, a "full" battery indication needs to meet both the desired voltage and the desired current flow and you get that with a standalone monitor such as a Trimetric for example as Booster noted above. You can set the charge indicator light to only come on if both 14.4V AND 2A or less current in criteria are met. That's close enough to fully charged for most folks.

In theory it would apply to lithium batteries as well but no one wants to or seems to recommend ever getting them to 100%. The charging goal with lithium is to get them to some point under 100% SOC like maybe 90% or 95%.
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Old 08-06-2019, 04:52 PM   #6
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Here is a quote from the link I posted above:

"We have not yet seen an Ah counter’s SOC be within 10% of actual tested capacity and most are far worse than a 10% error. Is this the fault of the Ah counter? No, it is the fault of improper use, lack of understanding and poor programming."

He explains in detail the reasons for this and how to avoid adverse consequences from it.
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Old 08-06-2019, 07:07 PM   #7
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Quote:
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In theory it would apply to lithium batteries as well but no one wants to or seems to recommend ever getting them to 100%. The charging goal with lithium is to get them to some point under 100% SOC like maybe 90% or 95%.
What does "in theory" mean? Who actually limits lithium batteries to under 90% to 95% SOC? I keep my computer, tablet and cell phone lithium batteries at 100% routinely and will replace them for functional obsoleteness before the batteries deplete. But they aren't RV batteries. I keep my Advanced RV batteries at 100% though ARV does stop charging and letting batteries drop down to 90-91% before charging begins again back to 100%. That was a software change about two years ago for me I suppose to answer "in theory" jitters or for people who think they are going to will their great grandson the RV. Before it was 100% and stop charging and then maintain 100% while plugged in and using your batteries. It is impossible to not go to 100% with their setup. Going on 4.75 years there is imperceptible drop off in performance and that is with an older BMS since improved.

I suppose you can get anal in theory on this and limit your range but why? The in theory advocates also says you should maintain at 65% SOC optimally and that means less than half battery capacity usage. That's idiocy if you maintain 20-65% SOC for minimal gain. I am more concerned about the other end. Who else limits lithium battery usage at 20% SOC by automatically disconnecting, limits charging when batteries go below freezing and maintains battery temperatures optimally always about 41 deg. F?

OK, getting off my soapbox. I'm not an in theory guy, I'm speaking of actual use in having, I believe, the first ARV 800ah build of lithium ion batteries in 2014.
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Old 08-06-2019, 08:22 PM   #8
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"OK, getting off my soapbox."

Please don't do that too much.

I tend to agree and wonder why I'm limiting by 3hp ebike to 90% charge, switch selectable 80, 90, 100% SOC. It will out live me! Probably true with your B.

Tesla does limit charge and discharge. Then if there is some catastrophic event, they reprogram the usual charge and discharge limits.
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Old 08-06-2019, 08:44 PM   #9
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I recall davydd reporting more than once that Advanced RV stopped charging to 100% in favor of charging to 99% ............


Quote:
Originally Posted by Davydd View Post
..............So, I drive into a campground at a full 99% SOC. I was concerned that I could be at a 90% SOC but learned ARV has programmed only the shore power to charge to 99% and then let electrical usage drop it to 90% before starting charging again.
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.....................
Advanced RV is always tweaking their systems. Last year I had an upgrade to my 800ah lithium ion system. Instead of staying fully charged all the time I now float from 90% to 99% SOC. In other words, when I reach 99% charging shuts down until SOC drops to 90%. That is tweaking for longer life. Batteries shut down at 20% SOC but I have set Autogen (auto starting the engine) at 40% SOC but have never dropped to that on our road trips. So I am theoretically keeping my batteries in a middle SOC range. The Autogen SOC is programmable. I don't know what the upper end is off hand but I've programmed it to 90% SOC in my impatience to see if Autogen actually worked. I have once turned Autogen off to see if the batteries actually shut down at 20% SOC. It did.
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Old 08-06-2019, 09:22 PM   #10
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My understanding of lithium batteries for RV's is that it is not necessary to charge them to 100%. They are not diminished or affected by not charging to 100%. They might get diminished by charging to 100% but someone else who knows more can answer that.

Lead acid batteries do need to be charged to 100%. They benefit from it.

I would think that with either type of battery, the best type of SOC meter would reset when both voltage and current (amps) set-points are reached. If the SOC meter doesn't do that and instead functions as just a coulomb counter then it seems inevitable that it would become less and less accurate over time.
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