the dark art of battery readings.

[lexicon]

hi all

 

need some help.

so first to explain my set up.

 

4 110ah batteries 12v flooded lead/acid

235w solar at 30v with a 20a tracer mppt with display. 6mm2 cable from panel to controller 10mm2 cable to leisure bank. run of 3m from controller with inline fuse.

 

1600w sterling psw inverter 2m cable run 50mm2 cable.

 

so heres my question.

 

the inverter is on 24/7 powering a 240v fridge.

the inverter has a display showing voltage,

the solar controller has a display showing voltage and amps.

 

so the inverter shows a voltage of between 12.9 and 12.5 12.4 the lowest i have seen so far normally in the morning.

the solar controller shows sometimes 0.1v less than inverter.

 

is this due to the thinner cables and distance?

 

these readings are under load (as inverter always on plus some led lights.)

from research i gather they will always be lower than off load readings when the batttery has rested. is this correct?

 

and finally what sort of reading should I worry about in terms of batteries at 50% as some online charts show 12.2v as 50%.

 

thanks for any advice.

[Grace and Favour]

Yup 12.2v is 50% charged on a 12v battery

[lexicon]

so if I am seeing 12.5 to 12.7 on the inverter i can breathe relatively easy.

[DeanS]

Yes, you've got nothing to worry about. If you take a reading in the early morning, that's probably a good reflection of the state of the batteries at their worst....before the solar kicks in for the day, and after a long load time...and under load.

[grahame r]

12.2v is 50% State of charge when the batteries are under no load. As your inverter is on constantly this will not be the case and a lower voltage may be indicated with the batteries still above 50% charged. I believe a smartgauge addresses this problem and gives a more accurate estimate of State of charge under load. Certainly 12.5 to 12.7 volts going into the inverter is healthy.

[nicknorman]

The 0.1 v difference you mention is probably just the accuracy of one or both meters.

 

On the subject of 12.2v = 50%, I think the load presented by an idling inverter and a few LED lights is insignificant. However its worth bearing in mind that if you are checking the bats in the morning after sunrise (which it will be at the moment unless you are an insomniac!) it doesn't take much charging current to substantially lift the voltage. So what looks like >50% SoC might in fact be just the solar panels lifting the voltage somewhat.

[Rich]

12.2v is 50% State of charge when the batteries are under no load. As your inverter is on constantly this will not be the case and a lower voltage may be indicated with the batteries still above 50% charged.

If the battery voltage is read without a period of rest, i.e. with no draw, would it be higher or lower than a reading taken soon after perhaps switching the domestic bank off just while the reading is taken? Or does it also depend on the amount of draw prior to switching off? Or, is the answer on a par with the length of a piece of string?

 

Richard

[DeanS]

Your last post confused me

If you wake up after the sun, then the voltage on the batts will already be lifted by the solar panels. I watched a shadow/sun line moving towards my boat....and the batt voltage was rising by 1V every few feet it moved closer. It was very exciting...LOL.

 

So...you should measure before sunrise.

 

If you measure in the evening.....make it late evening...as it takes some time for the effects of the days solar to settle....and until then, the battery voltage could seem higher.

 

At the same time though, if you are measuring with the invertor etc still connected, you could probably add on 1 or 2V to what you measure.

 

If you , however measured 30mins after sunrise or 30mins after sunset, the value you measure could be about 4-5V higher simply due to the solar panels.

 

One of my battery banks is sitting at 13.1V . By tonight it will measure about 12.6V without anything connected to it.

[nicknorman]

At the same time though, if you are measuring with the invertor etc still connected, you could probably add on 1 or 2V to what you measure.

Assuming the invertor is running nothing or a light load, more like 0.1 or 0.2v! Probably 0.1. But as mentioned, the solar panels will have a much greater effect (the other way).

[smileypete]

However its worth bearing in mind that if you are checking the bats in the morning after sunrise (which it will be at the moment unless you are an insomniac!) it doesn't take much charging current to substantially lift the voltage.

 

That's why battery reading is a 'dark art'...

 

Ba-doom tish!

 

(goes off to fetch coat...)

 

cheers, Pete.

~smpt~

Edited May 21, 2013 by smileypete

[trackman]

Get a Smartgauge, mine is brilliant & removes all the guesswork.

[lexicon]

It's on my to do list.

But recent more important repairs to gearbox/engine means the funds need to heal