BATTERY LIFE

[bc toys]

I was reading something here while back that you don't want your battery to get below 50% well can anyone tell me what the voltage will read at 50%
I read where a guy let his voltage go down to 11.4 volts that seems low to me unless he is getting below 50% I've never had mine get below 75% yet sense
I got the solar system

[lfhoward]

This battery webpage has almost everything you need to know about RV (and ttt) electrical systems, including a power chart. They say 12.06 is 50% charge.

http://rvroadtrip.us/library/12v_system.php

Of course that wouldn’t be 12.06 volts under load, but after the batteries have had a chance to equalize.

[friz]

Same for me. I have an AGM battery and my cut off is 12v.

Sent from my SM-G930V using Tapatalk

[John61CT]

Totally varies with the chemistry type, specific model, resting time isolated before measuring, and then changes over time as the capacity declines as the bank ages.

Bottom line is, voltage is a very inaccurate guestimate of State of Charge.

Even the best (expensive) battery monitors can easily be off by 8+%

Best combination IMO if you can justify the investment is a Merlin/Balmar SmartGauge + Victron 702-BMV.

Otherwise just the SG.

Search http://marinehowto.com for "battery monitor".

[Socal Tom]

I try to keep mine above 12.2. John is correct, voltage isn't a very accurate measure, but just about every one of the charts I've found says 12.2 should be ok. However, I don't actually start to panic until it gets below 12.
Tom

[GuitarPhotog]

Totally varies with the chemistry type, specific model, resting time isolated before measuring, and then changes over time as the capacity declines as the bank ages.

Bottom line is, voltage is a very inaccurate guestimate of State of Charge.

Even the best (expensive) battery monitors can easily be off by 8+%

Best combination IMO if you can justify the investment is a Merlin/Balmar SmartGauge + Victron 702-BMV.

Otherwise just the SG.

Search http://marinehowto.com for "battery monitor".

Kinda tough to measure Specific Gravity on a sealed AGM battery, isn't it?

[Aguyfromohio]

Many years ago I was part of an engineering team for the main battery on the F-16.
We could only accurately know the state of charge of the battery by having a little micro processor measure the current entering and leaving the battery, and keeping a running tally of amp-hours remaining. We had a PhD electrochemist on the team, he assured us there is nothing you can measure at the battery terminals that tells you much. The more expensive battery monitors like those mentioned above measure current and perform that kind of amp-hour accounting. Our system had a lot of fancy correction factors for temperature, and rate-of-discharge, and cycle life history to improve accuracy.

Of course it may be worth asking the question “why try at all?”
If you are off grid and not able to charge, just use as little power as possible and when the battery is depleted that’s that. If you have a high quality smart charger wired in to the shore power circuit, plug in whenever you can and the battery will stay as charged as possible.

The Air Force cared because they had jet fighters not ready to go due to dead batteries that were being depleted by ground crews and failing early due to abuse. Our system disconnected the main battery at certain points when the ground crews ran the batteriy down by talking sports on the giant military radio or were goofing around too much raising the canopy up and down. They had to go outside to push a RESET button, and our memory data would tell their superior officers who was ruining the batteries and reducing the unit readiness. The average camping trip is less critical.

[Socal Tom]

Many years ago I was part of an engineering team for the main battery on the F-16.
We could only accurately know the state of charge of the battery by having a little micro processor measure the current entering and leaving the battery, and keeping a running tally of amp-hours remaining. We had a PhD electrochemist on the team, he assured us there is nothing you can measure at the battery terminals that tells you much. The more expensive battery monitors like those mentioned above measure current and perform that kind of amp-hour accounting. Our system had a lot of fancy correction factors for temperature, and rate-of-discharge, and cycle life history to improve accuracy.

Of course it may be worth asking the question “why try at all?”
If you are off grid and not able to charge, just use as little power as possible and when the battery is depleted that’s that. If you have a high quality smart charger wired in to the shore power circuit, plug in whenever you can and the battery will stay as charged as possible.

The Air Force cared because they had jet fighters not ready to go due to dead batteries that were being depleted by ground crews and failing early due to abuse. Our system disconnected the main battery at certain points when the ground crews ran the batteriy down by talking sports on the giant military radio or were goofing around too much raising the canopy up and down. They had to go outside to push a RESET button, and our memory data would tell their superior officers who was ruining the batteries and reducing the unit readiness. The average camping trip is less critical.

This is a less fancy, less expensive version I use
bayite DC 6.5-100V 0-100A LCD Display Digital Current Voltage Power Energy Meter Multimeter Ammeter Voltmeter with 100A Current Shunt https://www.amazon.com/dp/B013PKYILS/re ... KAb6P71JNN
I have it on a dpdt switch. When discharging it records watt hours used. Then when charging, I flip the switch, reset the guage and it tracks watt hours sent to the battery. It’s not perfect, but easy to see when I’m drawing more than I can afford.

[John61CT]

Best combination IMO if you can justify the investment is a Merlin/Balmar SmartGauge + Victron 702-BMV.

Otherwise just the SG.

Search http://marinehowto.com for "battery monitor".

Kinda tough to measure Specific Gravity on a sealed AGM battery, isn't it?[/quote]I never mentioned specific gravity, but yes that needs a rest period and per-model calibration for accuracy as well.

In this case I was talking about SG as in the SmartGauge SoC meter.

[John61CT]

there is nothing you can measure at the battery terminals that tells you much.

The SmartGauge does not count AH, the rest do, work off a shunt.

The SG is the most accurate of the bunch, gets more and more so as it gets familiar with the bank's use patterns. But the algorithms it uses are proprietary, have not yet been reverse engineered.

[John61CT]

This is a less fancy, less expensive version I use
bayite DC 6.5-100V 0-100A LCD Display Digital Current Voltage Power Energy Meter Multimeter Ammeter Voltmeter with 100A Current Shunt https://www.amazon.com/dp/B013PKYILS/re ... KAb6P71JNN
I have it on a dpdt switch. When discharging it records watt hours used. Then when charging, I flip the switch, reset the guage and it tracks watt hours sent to the battery. It’s not perfect, but easy to see when I’m drawing more than I can afford.

Certainly better than just using voltage.

AH counters get a bit more accurate for SoC readings when they require programming with chemistry type, coulombic charge efficiency, Peukert factor, and current residual AH capacity.

But they get less efficient over time, need regular "now Full" resetting and of course updating the AH capacity as that declines.

SmartGauge requires none of that and is also more consistently accurate.Only works with lead chemistries though, no good for LFP.

[KennethW]

I was reading something here while back that you don't want your battery to get below 50% well can anyone tell me what the voltage will read at 50%
I read where a guy let his voltage go down to 11.4 volts that seems low to me unless he is getting below 50% I've never had mine get below 75% yet sense
I got the solar system

Back to reality. The cost of a battery version camping fun. Yes taking the battery to low is bad but so is cutting the fun short be cause a battery is at 50%. Your call. I call fun and will buy replacement a little early.

Sent from my Moto G (4) using Tapatalk

[tony.latham]

I use a G.T. Power meter to keep track of both my amp usage and resting voltage. They work great.

For $18 they are a steal.



Here's a good analysis of there accuracy:



If I build #4, I'll put one on both sides of the battery to measure power going in from the solar panels and power being consumed.

Tony

[mustangcats]

I use a G.T. Power meter to keep track of both my amp usage and resting voltage. They work great.

For $18 they are a steal.



Here's a good analysis of there accuracy:



If I build #4, I'll put one on both sides of the battery to measure power going in from the solar panels and power being consumed.

Tony

Thanks for posting information about this meter. I just ordered one on ebay for $10.45 postage paid...it's coming all the way from China! It will come in handy for finding out just how many amps the Truck Fridge is drawing. Last summer when I was at Rocky Mtn. NP there were too many cloudy days and my sole 100 watt solar panel couldn't keep up with the demands of the fridge and the battery voltage dropped too low. Had to use the generator as a backup. The Truck Fridge was advertised as "sips the battery power", but I have found that is not the case in the real world.

[John61CT]

> Truck Fridge was advertised as "sips the battery power"

Compared to all the other fridge *types*

Peltier coolers
Mains powered
Absorption

All the Danfoss-style 12V compressor are very efficient,

and afaik the AH / day rates are quite similar to each other's