Van battery question

[Trebor English]

There’s obviously such a thing as “ignorance is bliss” but really, how is the average schmuck to know?

Most of the people I know who use 12 volts just don't know.   Most just kill the batteries with very few cycles using the RV only on weekends.  Generators seem popular.  You can easily tell the state of charge of a generator.  When the tank is empty it just quits.   Then you recharge it with a 5 gallon jug of regular unleaded.



The $200 electronic electron counters can be programmed with the necessary battery characteristics and then periodically reset when the battery is full.  With that they can give a good estimate of state of charge.  However, when your 100 amp hour battery is 2 years old it is no longer 100 amp hours.  Then you have to reprogram the counter to continue to get a good estimate.  Taking 50 amp hours from an 80 amp hour battery is not 50%.



I read the mainsail article about the other kind of monitor.  It only looks at the battery voltage as it goes up charging and down under load.  Heavy load has more effect than light load and lots of details of battery physics are programmed into it.  It doesn't tell you how many amp hours are left, it doesn't know.  It tells you what the state of charge is.  A 220 amp hour golf cart battery pair or a 35 amp hour wheelchair battery will have radically different capacity but knowing the state of charge, full, 50%, empty, is very useful.  That device takes several days of cycling to figure out the state of charge then it tracks.  The price is similar.



The following is based on presuming that you have a battery regularly used to run stuff and a solar charge controller and solar panel to recharge the battery on a regular basis.



What you can do besides flooded/hydrometer and expensive electronics is to manually mimic the expensive electronics with a $10 multimeter.  Then you need to charge the battery more than you discharge, without exception.  That means on day two of rain you must unplug the fridge, TV, and laptop.  Well, if the fridge contains $1000 worth of meds and a new battery is $100, then you need to make an appropriate choice.



What I do now is use the hydrometer once a month when I'm watering the battery.  What I did three years ago when I got the battery was to check it much more often.  That gave me confirmation that my charging regimen is working.  What I do now is I have my cheap PWM charge controller set to the right voltage, 14.2 in the summer, 14.6 in the winter.  The solar panel charges until the PWM starts going on and off holding the absorption voltage.  This is a really cheap charge controller.  It never decides that the battery is charged.  It never stops charging.  It never goes to 13.5 volts.  It just holds the programmed 14.2 volts.  Most battery killing is done by charging the battery to a level that is not quite high enough and then not holding that for several hours.  When the charge controller turns on the green light to declare the battery full, it stops charging.



A battery is full when you charge it at the absorption level and the current taken by the battery slowly drops to 1/2% to 1% of the battery capacity.  Then it is full.  With a sealed battery that's what you can do.  Your solar charge controller can't see how much current is going into the battery.  Even if it does measure what it is producing it doesn't know how much is going into the battery and how much is going into the laptop.  A solar charge controller doesn't know when the battery is full.



To measure that current you cannot use a multimeter amp scale.  You will burn it up or blow the fuse in the meter.  What I do is I use a shunt resistor that is 0.001 ohms.  One foot of AWG #10 copper wire is 0.000998 ohms according to Wikipedia.  So one foot is close enough.  A 13 inch piece of wire with crimped terminals is about right.  A half inch from each end (12 inches apart) you connect the volt meter, stab the insulation.  Set the meter to the lowest voltage scale.  One millivolt, 0.001 volt, across that resistor means one amp.  The Harbor Freight cheapest meter has a 200 millivolt, 0.200 volt, scale.  An AWG #10 wire can carry 30 amps continuously buried in a wall.  In free air it gets warm carrying 800 amps to start a diesel school bus engine for a few seconds.  If you have a wire from a battery to a bus bar you can make that wire AWG #10 and one foot long then you have 0.001 ohms from the bus bar to the battery.  Measure the voltage from one end of the wire to the other and divide by 0.001 to get amps.  Charging at 5 amps will show 0.005 volts, 6 amps, 0.006 volts.  This is crappy accuracy and horrible resolution and poor temperature stability but the price is right.


Amazon sells a $17 amp hour meter.  It has an external shunt resistor that is good for 100 amps.  These usually measure current in only one direction, charge or discharge, and are not battery state of charge monitors.  It's more accurate than a foot of AWG #10 wire and keeps track of amps over time to tell you amp hours. 



https://www.amazon.com/Multifunction-Bat...07T8GN61Z/



I have used a similar device between my solar panel and the solar charge controller.  That makes it reset every night when the sun is off.



Since I have a flooded battery I can add water.  With sealed batteries you have to be careful to not hold too high a voltage when the battery is full as that might cause water loss if the battery pressure gets too high and it vents.



You need to know what your charge controller is doing.  There are manuals online.  If you have a question you can PM me a link to the manual or quote from it and I can interpret what that means.  With an inexpensive meter you can see what the voltage is doing.  You have to pay attention several times per day until you understand what the system is doing.  Then you know what to look for.  It may be that charging at 14.4 volts by noon followed by 2 hours of that is what you need.  Then looking at your voltmeter daily at noon will be enough for you to know that it is working.


Some people have RV converter / chargers.  those are a different sort of beast.  Some will roast the battery, some will let it languish undercharged.  If it is a roaster, disconnect the battery and use the 12 volts from the converter.  When you go away from power, reconnect the battery.  When you get back to power, let it charge to 14.4 volts for several hours (6 or 8) then disconnect.



If you have a van starter battery that is not disconnected with a disconnect switch or cable removed you can use a diode like what people use to charge house batteries from the alternator except the other way.  Use your house battery and solar to charge the starter battery.  The diode will cause a .7 volt drop.  While your house batteries are getting 14.6 volts the van starter battery will get 13.9 volts.  That is not a good deep cycle charging but it will make it so that the parasitic loads come mostly from the house battery and the starter battery gets some charge.



Bottom line, get a notebook, write down date, time, and voltage.  Figure out what your charge controller is doing.  Observe charging voltage and duration.  Observe changes in loading.  Observe overnight low voltage before solar charging begins.  That'll be your first clue that your battery capacity is dropping, your load is increasing, or your charging is decreasing.