batteries . question number 12657.

[onionbargee]

How does one ensure ones battery bank never falls below the recommended 50% state of charge ?

 

Is there a simple or cheap option to sound an alarm when the voltage drops ?

 

How accurate is the battery voltage to determined SOC ?

 

I've got four T105's that cost a lot of money and I don't want to knacker them.

 

480 w of solar will be feeding them, when I get round to wiring the panels in.

[MtB]

480 w of solar will be feeding them, when I get round to wiring the panels in.

 

 

 

I think they'll be in deep trouble in the interim.

 

 

What counts as "50% state of charge" to you? This would seem the starting point to working out how to measure it.

[Paul C]

I think the worst thing you'd want, is an alarm sounding when the batteries are half way flat. It might happen at 4am, after all, with the fridge on overnight.

 

I don't think 50% is a hard limit anyway, the less you discharge the better and if you go below 50% its not the end of the world, or particularly damaging. The way to do it is to do a power audit (based on actual figures, not guesstimates), know the batteries ie know their capacity, and regularly monitor them using a decent battery monitor of some kind. In that respect, 480W of solar is meaningless because it would depend on the climate, time of year and loads on your electrical system.

[Richard10002]

In particular, T105s are proper deep cycle batteries, so if a "normal" open lead acid battery has an arbitrary limit of 50%, a T105 will be equally safe at a lesser state of charge - so no real need for an alarm, merely some kind of monitor.

 

The Smartguage seems to be the simplest and least confusing as it gives an unequivocal state of charge, (which seems to be accurate enough for our purposes).

 

I have a NASA BM2 as I want to know instantaneous Amps in or out, and cumulative Amphours. The BM2 has a stab at SOC, but it isn't particularly accurate, and requires a knowledge of capacity.

 

With a monitor, other than a voltmeter, in general, if the voltage falls below about 12.2V when little, or nothing, is being used, they are about half full.

 

This definitely isn't a precise science

[Keeping Up]

No 50% isn't a hard limit. It is just a guideline to use when determining how many batteries to buy. If your usage regularly exceeds 50% you will need to replace your batteries more often than is optimum, if you usage regularly fails to reach 50% you will spend more on your replacement than is optimum. Otherwise the figure has no meaning. Also it was determined at a time when battery prices, diesel prices, and solar panel prices, were very different from what they are today; therefore it could be completely the wrong number anyway.

[MtB]

This definitely isn't a precise science

 

 

Quite.

 

I prefer to save myself the grief so abuse/ignore my batteries mercilessly. Consequently I have to buy a new set for £140-ish every five years.

 

 

MtB

Edited May 21, 2015 by Mike the Boilerman

[Arthur Brown]

Basically a moderate set of batteries should last up to two years in service. If you abuse them that can go down to two weeks. If you are exceptionally careful then 5 years would be great and one report of ten years service has been made.

 

However no single short incident will cost a lot of life or prolong the life by lots. 50% SOC is a fair balance of life expectancy and charging cost and battery cost.

 

Rumour is around that regular periods just gently on solar charge does good things for battery life, certainly being left flat or partly flat for long periods is bad news.

Simply cramming the power in as fast as possible may not help cell life, but charging slowly costs a lot in diesel sometimes you save more on diesel that the batteries would cost.

 

Probably the best system is a shoreline powered clever charger, with some added solar and engine powered diesel charging when away from the home mooring.

[Phil.]

Smartgauge, and wire up the solar sooner rather than later.

[Richard10002]

Quite.

 

I prefer to save myself the grief so abuse/ignore my batteries mercilessly. Consequently I have to buy a new set for £140-ish every five years.

 

 

MtB

If your batteries are lasting 5 years, you must be doing something very right!

[MtB]

Solar panels and gas lights...

[nb Innisfree]

The 50% 'rule' has often been misinterpreted and become a mythical figure, a figure that has been seized on as an absolute and therefore simple solution. Folk like things to be simple but it just ain't so.

[onionbargee]

What voltage would you agree is the lower limit of discharge ?

 

I will go with that.

 

I'll get a little digital voltmeter off eBay, and just keep an eye on it.

[MtB]

What voltage would you agree is the lower limit of discharge ?

 

I will go with that.

 

I'll get a little digital voltmeter off eBay, and just keep an eye on it.

 

The lower limit I use is when the batteries refuse to work the fridge. I guess this must be about 11 Volts.

 

 

MtB

[keble]

From Tony Brooks' web site:

 

"Less than 12.2V = Very discharged

12.2V - 12.5V = Part charged

More than 12.5V = Fully charged."

HTH

[Tony Brooks]

But amplified on courses to try to explain what it really means. As someone said - people like it simple. I agree, no more than a working rule of thumb and nothing dramatic will happen if you go below 12.2 occasionally.

 

However, Trojan or any other lead acid battery more or less work with the same chemical reactions so all are subject to the sulphation problem so the lower you take a cruising boat the longer running is needed to recharge it so the greater the possibility of sulphation - whoever made the battery. Solar helps provide that longer charge so minimises sulphation.

[Arthur Brown]

The issue with voltage readings is that all references to battery voltages mean rested voltages, and that takes several hours of rest for the batteries no current in or out (inc solar!) A battery rested may read 12.2v put it on a charger an it might immediately read 14v but it hasn't suddenly become fully charged, that takes amps and hours. A battery reading 12.7 may have a starter load applied and may read only 10v instantly it isn't flat but under heavy load and will recover some of those volts as the starter load is removed.

[Tony Brooks]

But for the purposes of assessing day to day state of charge rather than being very accurate the almost no load voltage reading taken when charging has stopped and after a small to moderate load has been applied for a while will give a usable figure. Depending upon ones interpretation of "almost no load" it may under estimate the state of charge.

 

Solar causes a big problem in this respect because at this time of year it is present for 16 hours a day plus so being practical one needs to do the voltage check at night. I do not find it onerous to check the fridge is not running and use a torch to check the battery voltage ( have a decent voltmeter) just before I go to bed. This is probably just as reliable as using an amp hour counting meter. It does not take many night's readings to assess if you are gaining charge, loosing charge or just covering the days loads.

[valrene9600]

But for the purposes of assessing day to day state of charge rather than being very accurate the almost no load voltage reading taken when charging has stopped and after a small to moderate load has been applied for a while will give a usable figure. Depending upon ones interpretation of "almost no load" it may under estimate the state of charge.

 

Solar causes a big problem in this respect because at this time of year it is present for 16 hours a day plus so being practical one needs to do the voltage check at night. I do not find it onerous to check the fridge is not running and use a torch to check the battery voltage ( have a decent voltmeter) just before I go to bed. This is probably just as reliable as using an amp hour counting meter. It does not take many night's readings to assess if you are gaining charge, loosing charge or just covering the days loads.

Exactly what I have done for the past 9 years. Doesn,t take long to get a good idea of what is normal and to also spot a problem.

Having solar is in my opinion a must for a liveaboard.

[Psycloud]

I'm using this - it doesn't sound an alarm but it keeps me informed.

 

http://www.seamarknunn.com/acatalog/nasa-clipper-bm-1-battery-monitor-24v-100-amp-8024.html?gclid=CJzokYOq1cUCFTLLtAodrhUAbA#.VV8eD5NViko

 

EDIT: Should have checked all the posts. Has already been said - sorry.

Edited May 22, 2015 by Psycloud

[smileypete]

Posted this to a recent topic, added a bit to it:

 

Batt voltage under discharge should recover above 12.2V when little or no loads are on. If not then the batts are around half charged or less, and for typical leisure batts it's time to recharge.

 

For tractions the batt voltage should recover above 12.0V when little or no loads are on. If not then the batts are around 20% or less, and it's time to recharge.

 

When charging, batt charging current should tail off to a steady low level as the batts approach a full charge with the correct charge voltage. Typically this is 1-2% of batt capacity.

 

For batts in daily 'off grid' use, they should get a full charge at least weekly. Proper 'semi traction' batts also need periodic equalisation charges to the correct eq charge voltage too.

 

cheers, Pete.

~smpt~

Edited May 22, 2015 by smileypete