Battery Charge/Depletion Management: Help Needed

[riggyk]

Guys,

As is the case with my perpetual naivety on all this stuff, I need some pointers/help with battery management and charge/depletion monitoring devices....

I've bought this digital voltmeter..... (https://www.amazon.com/gp/product/B005UWD2J4/ref=oh_aui_detailpage_o03_s02?ie=UTF8&psc=1)

Is it as simple as just throwing a hot/lead wire from the voltmeter and tapping it to one of the battery posts in the bank and reading the voltage? ~ (slightly less than) 12V = fully charged? Anything below that indicates depletion? 

I'm curious if a single voltmeter is sufficient or if there's a different/ideal way to go about reading your battery charge status.... I'm scrapping the fridge in my setup and now running only personal charging devices off the inverter and low draw 12v items off my setup, so I'm thinking my energy demand will be greatly reduced and not much of an issue. (Now have a 600W Inverter that might be overkill! BLAH)

 

[Trebor English]

Just the voltage is not really a good way to tell state of charge.  If you let the battery rest for several hours hot, a day when cold, longer below freezing, the voltage is a good clue.  12.2 and below is drained.  With a flooded battery a hydrometer will tell you actual specific gravity of the electrolyte.  That is a good way to measure battery state.

 

[Spaceman Spiff]

Assuming you have a lead acid battery:  A fully charge 12 volt battery is at a resting voltage of 12.7 volts, a 50% depleted 12V battery is at 12.2 volts, a battery with nothing left reads 11.9 volts.

You will need to wire your voltmeter to the battery: positive wire to positive terminal, negative wire to negative terminal.

Assuming you have a marine or deep cycle battery the actual voltage of the battery will lag the reading by between 2 and 24 hours.  this is a function of how charge migrates through the lead plates.  To understand the instantaneous state of charge of your battery you need to learn what the difference between instantaneous and resting voltages and adjust.  The voltage will initially read high after charging and low after discharge.

IMO a 600 watt (50 amp) inverter is way overkill and quite inefficient for small loads.

 -- Spiff

 

[jonyjoe303]

here's a state of charge graph. With lead acid you don't want to go below 50 percent to prolong the batteries life. Anything below 11.5 volts and your damaging the battery.

 

[SternWake]

Running an inverter, just to power USB ports to charge devices is very inefficient.

Use a 12v USB source instead.

https://www.amazon.com/Blue-Sea-Systems-Charger-Socket/dp/B01BVBRBXA

Voltage is not like a gas gauge. Voltage will read lower under load, and higher when charging and higher after charging, and there is a time delay before it settles to give an accurate reading of state of charge.

Close enough is another matter.

I personally despise those voltage/ state of charge charts.  My AGM battery fully charged and rested reads13.06v 
.  Some chart saying 12.7v is fully charged on this battery is insulting to me, and my battery.

Recharging in the 12.2v range is wise.  10.5 volts is considered 100% discharged.

It is not as if depleting to below 50% is instant death for the battery.  I do it all the time, and have well over 550 deep cycles on mine.  The trick is to recharge promptly and fully regularly.  If prompt and full is achieved then the 50% chicken little sky is falling mark  can largely be ignored.  going below 50% and then only recharging to 85% will quickly degrade the battery capacity.

If you really want more precise readings of battery state of charge then an amp hour counter will monitor how much juice is flowing in or out of battery.  these require a Shunt be placed on the (-) line and nothing goes to battery(-) that does not flow through shunt.

These shunt based amp hour counters are generally in the 200$ range, for quality ones, but there are cheaper options recently available whose accuracy and function i cannot personally attest to.

I recommend a simple ammeter, so one can at least know how much amperage their devices pull, and how much amperage the battery is accepting from the charging sources at the voltage the battery is being held at, at that moment.

A person who watches a voltmeter and an Ammeter  while both charging and discharging can get an excellent idea of the state of charge of their batteries as the cycles accumulate, and notice performance loss/ capacity decline as the battery ages.

In general, All you need to do is keep voltage above 12.2v, and recharge it promptly and occassionally  truly fully to get acceptable service from a battery.  Good to great service life of a battery requires full recharges more often.  Battery capacity is like a balloon whose skin becomes less and less elastic whenever it is less than fully inflated, so keeping it inflated as much as convenient is wise.

The inverter is a great battery depleting tool.  A necessary evil in my opinion, but a laptop DC to DC car adapter is 10 to 25% more efficient than using an inverter to power the ACDC power brick the laptop came with.  I rarely use either of my 2 inverters, one 800watt msw and one 400watt PSW.

The issue with laptop car adapters, is the ciggy plug will, at some point, fail,.  And  it will fail faster when it is asked to pass more than 60 watts.  Here is a solution to that:

https://www.amazon.com/Blue-Sea-Sys...mr1&keywords=blueseas+12v+plug+and+receptacle

 

[riggyk]

Sorry, I should've clarified too however, although the majority of the charging is done to personal items.... they aren't all USB types. My phone yes, but then there's a laptop, and a Bose Sound Dock charger, in addition to a string of LED lights that are going off the inverter as well. Maybe a 02 cool fan adapter as well. Plan to incorporate a fridge into this picture at some point in the future too. So yeah...maybe the 600w setup is a bit overkill but there's slightly more to it than just USB device at present. How would the ammeter get worked into the circuit with the voltmeter staying on the battery posts?

 

[Trebor English]

     How would the ammeter get worked into the circuit with the voltmeter staying on the battery posts?

Get a multimeter.  Harbor Freight has one that is usually about $5, sometimes free with a coupon.  Amazon has a bunch around $10.  They come with a little manual  Study that.  It will answer many questions.

To measure volts and amps at the same time you need two of them.