best way to check battery for state of charge

[doublegregg]

by SOC i hope i'm using the correct term. i'm just figuring out my solar system, used wheel chair batteries (mk gel, 51ah). i tried charging my battery on my solar system, which said the battery was full and was charging at 1.2 amp?.  i thought the battery was fully charged, and tried running my small fridge on it. a fridge the size of a medium large cooler. the voltage dropped to 12 v within minutes, and i stopped. now, a half hour later the voltage has popped back up to 12.5.

should i have continued charging on the solar system???? i'm wondering if it needed to be in its final stage of charge, which i've heard can take...hours. is there a way to accurately monitor the battery --- more than volts, which seem to fluctuate wildly. i've heard people mention a battery shunt, or other devices sold by companies......... do they actually work, or are they money tossed out the window at 80mph..?  should i have let my battery continue to run, despite the volts dipping to 12?

it would be great to have a way to accurately know how much battery power i got -------------------- simply by looking at a gauge.... voltage seems like just a guess...........at best.

thank you

 

[B and C]

Let your solar system fully charge the batteries by leaving it connected. Is there a reason you keep disconnecting it? The green, I am full is a poor indicator too. I think gel batteries are considered charged like most other LA batteries when charging drops to 0.5 amp of capacity. This is 0.25 amps in your case.

Another factor is your batteries are used so you really don't know the capacity they have.

The Peukert effect has a say in fast drawdown of a battery too. This is where the voltage of the battery is rebounding after a drawdown.

A true battery monitor that is shunt based will give you real world readings but you are supposed to start with a full battery. Leave your solar connected for a couple of days or put on a shore charger. A couple of hours won't cut it.

 

[Trebor English]

Just looking at the voltage is a bit of a guess.  You can get a smart battery monitor for about $200.  That will watch the amps going in and the amps going out over time and keep track and give a good estimate.  The state of charge depends on the prior history of current in and current out.  The voltage depends mostly on what you are doing to the battery right now.  The resting voltage, no current for several hours, is more meaningful.

If you get a flooded battery you can use a hydrometer to measure the density of the electrolyte.  The amount of sulfur in the sulfuric acid is a measure of how charged the battery is. 

If you actually have a gel battery as opposed to the more common AGM wheelchair batteries you need to be careful not to charge it too fast.  It can form bubbles like a flooded battery but since the gelled electrolyte is immobilized the bubbles don't float away.  The portion of the plates involved in the bubbles is no longer participating. 

You don't say what the charging voltage is at 1.2 amps.  For the battery to be full you must hold the absorption voltage, 14.4 volts, until the current accepted by the battery drops to 1/2% of the capacity, about 0.25 amps for a 50 amp hour battery.  For gel batteries avoid charging faster than 10% of capacity, 5 amps for a 50 amp hour battery, to avoid the bubbles damage issue.

 

[desert_sailing]

I picked up this voltage reader at a car store.
https://www.walmart.com/ip/12V-24V-...er-Voltage-Meter-Battery-Gauge-Blue/553591318

It came with ZERO instructions. The salesperson said it would tell me what remains in the batteries. He said it also would stay on 24/7 once it is wired as it has no off switch and gave me a toggle for it.

Would anyone be able to tell me..what gauge wire should I use. I was thinking of mounting the display and having alligator clips running to the battery and occasionally checking the remaining juice. The charge controller is already telling me what is going out... (I think).. I have no idea as of now to know what I have remaining.

 

[doublegregg]

b&c
i disconnected it cause this is the first time charging for me .... i will take the green full batt light w a big grain of salt. i'm unclear on the math involved of my 51ah battery, the 0.5 amp of capacity and how that works out to .25amp?

trebor
i don't know if i want to spend $200 on a smart battery monitor ---- is that something you have, or were you just answering my question? i don't think i'm getting a flooded lead.... prob from listening to various videos, including bob wells. my batteries say gel... i'll have to limit my dokio panel to 5 amps when charging ---- it can charge at 16 amps. that seems inconvenient...

so i'm just 'experimenting' sort of with these batteries. i have two; they're used... the guy told me he thought they had 60% capacity. so they're my learner batteries, or maybe they'll be useful. i don't understand enough about this to know. limiting charging in the early stages to 5 amps doesn't seem good.

any suggestions on what type of battery i should ultimately get? my main load will be, i think, my iceco fridge - 50L... the rated dc current is 4.5/2.1 amps.... i'd prefer a lighter battery, such as these..... i'm not full timing and park the van out on a city street... so, it's not easy to hook up solar, etc to it...

and are there any good sites for batteries... they're really complicated.

thank you...

 

[bullfrog]

There are years worth of attempts to explain how to custom build a system that will do for a specific individual what they are wanting to do but there are limitations of cost, convenience and efficiency. Best to start your education by researching the forum then asking knowledgable questions. If all else fails go to someone’s blog or video that has done what you are trying to do. One reason the RTR and builds are so popular is seeing makes it easier to understand or at least copy something that works. Pictures of tags or specs and components as well as drawings make for much better answers.

 

[B and C]

b&c
i disconnected it cause this is the first time charging for me .... i will take the green full batt light w a big grain of salt. i'm unclear on the math involved of my 51ah battery, the 0.5 amp of capacity and how that works out to .25amp?

Capacity of 100 amp hours at .5 capacity would be a 1/2 amp.
Your capacity is 1/2 of 100 AH and is thereby 1/4 amp.
When charging at 14.4+/- volts and the amps that the battery is accepting is .25 amps is when your 51Ah battery is considered full.  Your voltmeter will not tell you the amps.

Flooded lead acid batteries come in three flavors, gel, AGM and liquid acid.
The acid in a gel battery is in a gel format and not a liquid.
AGM batteries are Advanced Glass Mat where the acid is in the mat.

Liquid acid batteries have caps that can be removed to check liquid level and specific gravity.
AGM and gel do not have removable caps and if charged too fast, they will bubble and off gas.  If this happens, capacity is lost as the electrolyte cannot be replaced.

I have a high dollar Trimetric TM-2030-RV to monitor my batteries.  I have an investment in my batteries and this lets me know what I am doing to them.  I don't want to ruin them by over discharging or under charging.  Having said that, there are much cheaper versions out there that will use a shunt.  I cannot recommend one because I haven't used one.

If you want a quick charging battery, LiFePo4 is probably the way to go.  There are complete boxes out there called solar generator (misnomer, they don't generate anything).  Hook a solar panel to them or plug into a 12V power 12V DC/120V AC outlet and go.  They are simple as all the engineering has been done for you.

 

[highdesertranger]

I also have a trimetric TM-2030-RV. once set it tells you all kinds of info but the two i find I use the most are.

1. battery % of full. example, it gives you a reading of like 82%, that means your battery is 82% full, with 100% being full.

2. amps in and out, at a glance you can tell which direction power is flowing into the batteries or out of the batteries. example, right now (night time)mine reads read -5.4amps. that means that 5.4 amps are flowing out(discharging) from my batteries. when the sun comes up in the morning the -negative will turn to positive and I can see exactly how much power i am getting from my solar panels.

it removes any guess work from the equation of trying to figure out the state of charge(SOC) of your batteries. it's the greatest invention since sliced bread.

highdesertranger

 

[Trebor English]

trebor
i don't know if i want to spend $200 on a smart battery monitor ---- is that something you have, or were you just answering my question?  i don't think i'm getting a flooded lead.... prob from listening to various videos, including bob wells. my batteries say gel...  i'll have to limit my dokio panel to 5 amps when charging ---- it can charge at 16 amps. that seems inconvenient...

I do not have a smart battery monitor, just answering your question.  I only paid $80 for my battery so paying twice that price to monitor it seems wrong.  I have a flooded battery and occasionally use the hydrometer to measure the electrolyte density.  Mostly I have a regular charge and use regimen and that is keeping the battery happy according to the hydrometer.  I repeat it. 

Search on Amazon for "DC amp volt watt energy meter" and you will see several inexpensive, under $20, meters.  These will show how much current you are putting in to your battery or, by reversing the wires, how much you are taking from your battery. 

These are not battery monitors and do not show the state of charge of your battery. They are cheap meters that let you measure what you are doing to your battery not what the state of charge is.

I have one like this: 
https://www.amazon.com/Proshopping-Precision-Analyzer-Checker-Measurement/dp/B07Z8XJ71K/

It says it's good for 200 amps but the wires sticking out of it are only good for 20 or 30 amps.  The 100 amp bayite and DROK brand meters have an external shunt resistor.  That mounts at the battery and the full battery current flows through the shunt resistor with skinny wires going to the meter display unit.  My meter has an internal shunt resistor and the full current passes through the meter. 

With these meters you can actually tell when your battery is getting 14.4 volts and the current has dropped to 1/2 percent of the rated capacity.  Since you can't use a hydrometer and don't have an expensive monitor what you can do is fill the battery regularly and then keep track of how much you use.  Step one, fill the battery, depends on charging it until it is full.  You only know it's full by charging it until the current taking tapers off. 

Different subject, you said you have two of these batteries.  Put them in a parallel configuration.  That makes your battery 102 amp hours so your max charging current is 10 amps not 5.  If you can find the battery manufacturer web page with actual details about your batteries they may recommend 13% or 15% of the battery capacity, 13 or 15 amps.  My warning about 10% was due to many people wanting to charge AGM batteries at over 20%.  Gel batteries are different.  If you treat them like flooded batteries or AGM batteries they will not do well.  Lacking any other guidance from the battery manufacturer, 10% is reasonable.

"the guy told me he thought they had 60% capacity."  Some people would advise treating the battery like it is a 60 amp hour battery rather than 102.  Only take 50% of 60 amp hours rather than 50% of 102.  Only charge at 10%, 13%, or 15% of 60 amp hours not 102.

You say your dokio can charge at 16 amps.  My 100 watt panel usually delivers 5 to 6 amps.  With the above mentioned meter you can know what you are doing to the battery rather than presuming that you will put 16 amps into it.

If you do these things and your pair of used batteries actually run your fridge for several months then you will have learned a lot about running your own 12 volt electric utility tuition free.  Some (many) people recommend discarding batteries when their capacity drops to 80% of the rating when new.  As with most things, my opinion is different.  I think that if the capacity is still sufficient to do what you need, keep them in service, keep the money in your wallet.

 

[Spaceman Spiff]

i don't know if i want to spend $200 on a smart battery monitor . . .

I got a $200 education on the way my solar/battery/load system works.  That I got a computer that gives me good information on the status and health of my system is a bonus.  I am on year 6 on my GC2s and they still hold 12.7V, with a S.G. of 1.275±.003.

There are cheap one-way amp counters on Amazon.  I don't have any knowledge on how good, accurate, or durable they are; one generally gets the quality one pays for.

 

[maki2]

if you just want a quick reference readout for amps and volts you can get a small LED display such as this one from Powerwerx. You can use it with your toggle switch. https://powerwerx.com/panel-mount-dc-amp-volt-meter

But if you are not charging your batteries very often keep them in your house on a suitable quality charger. Make it easy to put them back in place and secure them. Get a correct size Anderson plug from Powerwerx so that it is an easy connect and disconnect with fittings that stay attached to the battery and the mate attached inside the vehicle.

The battery charger I just bought has smart charging circuitry in it. It cost me about $80. But it can handle charging my 160 amp hour battery while I am working on my build where I have shore power.

 

[B and C]

Here is a cheaper shunt based meter that seem to have good reviews.

 

[doublegregg]

thanks, guys, for the gr8 and detailed info.... really appreciate it!

so i did one battery... since i don't no how healthy either battery is, i just charged the one. all day today, with my dokio, over 9 hours, the voltage was always 18 to 19, and amps went from 1.5, for two hours, and then 5 to 6 hours of .5, then .3, then .1 amps. which i assume is float. i think the initial two hours was what they call boost charging........ epever (my controller) has a) bulk b) constant, which includes boost, i think, and c) float...

so it seems like my epever is charging correctly? so far... i'm sort of not sure what to do with the one battery, other than check the voltage later... finishing voltage was 13.7, but half an hour later it's 13.1. if the voltage remains ... uh... above 12.6 should i test it on my 50L fridge? the first time i tried this, the voltage dropped quite low pretty quickly, but the battery was probably not fully charged....

i'll keep the trimetric in mind... so, it's actually accurate, in terms of % of charge?

happy holiday.............

 

[Trebor English]

If the dolio voltage feeding the epever controller was 18 to 19 then that's the way it should work.  The voltage at the battery would be actually relevant.  Knowing the voltage at the battery would be necessary to answer the question "so it seems like my epever is charging correctly?"

Knowing the voltage at the battery would be necessary to know if the battery got full.  Knowing the voltage at the battery hours later doesn't tell you whether the battery got filled.

When chargers, solar or other, decide the battery is full they stop charging and maintain a voltage in the neighborhood of 13 to 13.8 volts.  "then .1 amps. which i assume is float" could either mean that the charge controller dropped the voltage to stop the charging, the usual meaning of float, or the battery is full if the applied voltage remained at 14.4 volts and the charge current tapered off to a very low rate. 

You could apply the 50 liter fridge as a test load.  Since you have two batteries, connecting them in parallel would cut the current at each battery in half.  Why not do that?  You could have charged them both at the same time but didn't.  Again, why not?  Could you charge the other battery then load them both with the fridge?  Would both batteries running one fridge be representative of how you intend to use the batteries? 

Car headlight bulbs are excellent as a load for battery test purposes.  They are a moderate load, about 5 amps each, and they stay on.  Refrigerators go on and off and the current varies with refrigerant temperature so you don't really know how much you took out of the battery.  An inverter and a 3 way lamp, 50 watt, 100 watt, and 150 watt, are also handy for testing batteries.

The volume capacity of the fridge is 50 liters.  How much current does it take is a bit of information that is important.  Does the machine have a sticker that says 60 watts?  Did the amp meter indicate that it takes 6 amps?  To be a meaningful test of the battery you really need to know that.

 

[Spaceman Spiff]

Knowing the voltage at the battery would be necessary to know if the battery got full.  Knowing the voltage at the battery hours later doesn't tell you whether the battery got filled . . .

???

[size=small][font=Tahoma, Verdana, Arial, sans-serif][font=Arial, Helvetica, sans-serif]"Lead acid batteries are sluggish and cannot convert lead sulfate to lead and lead dioxide quickly during charge. This delayed action causes most of the charge activities to occur on the plate surfaces, resulting in an elevated state-of-charge (SoC) on the outside.[/font][/font][/size]

[font=Tahoma, Verdana, Arial, sans-serif][font=Arial, Helvetica, sans-serif][size=small][size=small]A battery with surface charge has a slightly elevated voltage and gives a false voltage-based SoC reading. To normalize the condition, switch on electrical loads to remove about 1 percent of the battery’s capacity or allow the battery to rest for a few hours . . . "[/size]
[size=small]https://batteryuniversity.com/learn...ace_charge[/SIZE][/size][/font][/font][/size]

For deep cycle lead acid batteries this requires a rest of between 6 and 18 hours, depending on the thickness of the lead plates and other factors.  This is needed to allow the charge to migrate deep into the plate.  One can learn what this elevated voltage is for a particular battery and adjust, but the reading is not giving a true voltage right after charging.

Loss of charge on a rested battery is also a red flag to the condition of the battery.

 

[doublegregg]

fir the battery, 24 hours after taking it off the charger it's at 12.97 v. it was at 13.1 right off the charger... does this mean the single battery is good, or i guess simply imply its' probably good... maybe it needs an actual load.

i'll hold off on the measuring meters --- they're an extra thing to learn. i'll keep in mind the trimetric - it sounds solid...... (it's expensive enough)

for my iceco 50L fridge, which will be my main load, it says it takes .4 amps of AC... is there a way to calculate this to 12v dc, or do i need to hook it up to measure dc volts... and is there a formula to calculate what size battery i'll need for it.... i guess i'm assuming the fridge is on... 1/4 of the time during the day? and i'm guessing i'd want enough battery for three days of clouds..... sorry if this sounds vague.

trebor, i didn't charge both cause i really don't know the quality of either; they're used, but should have some use remaining. so, i'm learning on questionable batts. for charging with my epever, the voltage remained at 18 to 19v throughtout, and i measured every hour or so. i couldn't think of any way to test two unknown batteries simultaneously. i figured if one was weak, or both, it'd be difficult to tell what the problem was if i used both batteries.

 

[B and C]

On the amazon site for the 40L, someone stated 4.5 amps at 12V DC. Yours should be no more than 5 amps. At your stated 1/4 run time you will use about 30 amps in 24 hours. This is 1.25 amps per hour.

After a full charge and the battery resting at least overnight it sounds like your battery is holding a charge. Only putting it under a load will tell the capacity.

You're doing it the right way, one at a time till you know what you have. I would start charging the other battery now.

Are you using the voltage off the epever or are you using a seperate meter? 18-19 volts sounds like panel voltage, not battery voltage.

 

[doublegregg]

i was quoting panel voltage.... battery voltage, which the controller seemed to monitor well, was 13.8 to 14.2. ty for the calculation......... 30 amps per day............ just for the fridge.... so these really are learner batteries? since the total amp hours between the two is 51x2 or 102 ah.... and you can discharge gels only .... half? if these were healthy batteries they'd be pretty minimal?

i guess i'm wondering about lithium, since they're lighter, can be discharged deeply, and recharge quickly. but they cost, and the cost of possibly damaging them, and the cost of them getting stolen are all concerns. mainly the last two, though. the initial cost - i'd pay if if not for the other two issues. anyway, it looks like once i figure these out i'll be replacing them.....

i'll charge the 2nd one tomorrow........

 

[B and C]

I think with a PWM controller, the panel voltage has to be above battery voltage or the batteries won't charge. I could be wrong on this as I have a MPPT controller. So giving us the panel voltage provided no real information on what was happening to the battery. The voltage at the battery is what is important. When charging a battery, you will notice the battery voltage climb until it reaches 14.4 +/- volts and hold that until the amperage going into the battery tapers to .5C where the voltage will fall to somewhere around 13.2-8 volts depending on LA battery chemistry.

Discharge only half. The shallower you discharge a LA battery bank, the longer the batteries will last. I generally discharge to around 80-85%. This gives me another day if I need it. I do have an onboard generator to help get past the rain days.

I've been pondering lithium myself because my batteries are getting old, probably 10 years now but they have not been in constant service. As my batteries are inside (where it is warm or cool), I do not worry about theft at all. You don't want lithium to freeze or get too hot either. I would have to change my converter and get a battery to battery charger to replace my existing battery isolator.

 

[doublegregg]

well, i'm looking at my current batteries (two, used gel, 51ah each) as a learner set... probably... 

so i put one of them, which i charged fully, on my 50L fridge (on an inverter). i left it connected for 6 hours. the voltage gradually dropped to 11.61. i disconnected it.  i'll see if the voltage rebounds... so, it works......... for awhile........... 

it seems like knowing the total amps going in and out, over time, would be good to know. do controllers keep track of that? i know a couple people here (hdr) mentioned a trimetric device, and then there are various cheaper devices, but don't no if those just give a reading of what is happening right then, rather than a log. soundslike the trimetric keeps a total. what is a shunt? 

and now i'm starting to think of what type of battery i'll eventually get. i'm thikning agm vs liion phosphate... but leaning to agm because of the cost of a 100ah liion, which is like $900. as i said, i'd be worried about them getting stolen, or me doing something to break it... windy nation 100ah agms are less than $200 ea. i'm thinking if my fridge needs approx 30ah a day, over 3 days that's 90ah, so i'm thinking of trying to get 200ah of battery.... i'm figuring agm will be half the price of liith ion? i'm leaning away from flooded lead acid cause of the spillage potential, and off gassing in my living compartment.......... i've read they can last longer if you take care of them.... i think i might not want that burden.

i do like the size of my current batteries. securing four of them, since i want 200ah, and wiring them, might be harder than two large 100ah batteries. but i'd prefer working with lighter batteries. i think they're 40#. 60#, for the 100ah agm's, is heavy.