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Battery Monitor?

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The BEP one works well, PROVIDED it is correctly programmed. Make sure you get the correct Perkets co-efficient from your batt supplier, so you can set it up right. It should auto-reset the capacity every time it is fully charged, so as it does not slowly get out of "sync" with the batts.


I have had more than 10 years out of the house batts on Island Time, much, I think, to do with looking after the batts with the info provided by the monitor...


Some people I've talked to reckon batt monitors are no good, but when you get down to it, mostly they are primitive models, or are incorrectly set up. I like them...

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and as a secondary question,  budget is there now for either a smart charger and installation, or replacing the current lead acid batteries (which are 2 years old) with AGM batteries,  which one do you think will be the best spend of the money?  All this is leading up to eventually replacing an old electric fridge.  So eventually will do it all but probably over 18months.


Current alternator is a 60amp valeo somthing

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I'm sure IT will chime in but my personal experience is AGM wont stand up to abuse as well as good lead acid batteries. If your lead acid are still good AGM wont necessarily be an upgrade. 

Are you talking smart charger via 240 volt or one on the alternator? Both are a good investment IMHO.

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1st let me say I am not a Battery specialist, what I know comes from years of use and a technical interest only. Take or leave my ramblings as you see fit! :-)


I think that if the Alternator is internally regulated, or does not have a smart charger, then the biggest single improvement you can make is to fit a proper regulator - 3 stage, temp compensated.

If the Lead Acid batts are ok, I'd leave them alone...


Willow, I'm interested in your comments re AGM. The Batts in Island Time (now due for replacement) are over 10 years old, and they are AGM/Gel hybrid batts. I think that the main issue people have with batts, including AGM, is abuse causing premature failure. The abuse is usually incorrect charging voltages/procedures,  and/or over discharge. Remember that for long life a batt should not be discharged more than 50%. It should be kept in a highly charged state. Read the manufacturers recommendations for charging voltages - don't guess, OR take ANYONES (except the manufacturer)  word for the right voltage! If you do this you should get many years from your batts, AGM or Lead Acid, as far as I can see. It is common to see failures any time after about 2-3 years, or even less if there is a serious issue with use or charging.


I really had good service from my AGM hybrids, including 3 years live aboard, and they are almost 11 years old now. It has only been in the last 6 months that the performance drop off has been noticeable. I have wondered if my use of the shore power (2 years now) system was the cause - if you have a smart charger (shore or alt), make certain that the voltage settings match the batts needs - not just he label, but check what it's actually doing!


I'll soon be considering if I will replace these batts with the same type, or move to something newer - possibly lithium. When I get to that point, I'll post some stuff on here...

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I've been wondering the same thing.  I started researching this but decided I had other more basic boating demands to resolve first.  Got as far as for calculating typical demands, storage and and charging requirements.

But it does seem to me that you could get rid of the start battery and rely on one house battery, for my little demand anyway.  The Li-Io batteries in Ferrari's link suggest that they are capable of both deep cycle usage down to 100% and CCA for starting.  But sh*t they are expensive - setting up a battery system as IT describes, with vigilance, resulting in a 10 year life for a good battery would still be hard to beat.

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Yeah, Lithium is good but expensive. Smaller, lighter, deeper cycle, but for me the real advantage would be charge rates - REALLY fast. Not sure I can justify the expense this time, but this next lot of batts is likely to be the last AGM hybrids - in another 10 odd years I bet lithium will be cheap!

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i've got a pretty basic system of 2 gel house batteries (2nd hand 2yo? american MK mobility scooter batteries that had passed a discharge test)


and a new lead acid car battery for starting the new 2cyl diesel


the house batteries are kept topped up by 50watt solar regulated by a simple/ dumb marlec regulator that apparently charges to 14.4v. it has a 3 color led to show charge/regulate and house battery status


the MK gel spec sheet says they are ok to charge to 14.6v


but with engine running the voltmeter on the old furuno gps showed the alternator to be pushing out higher than that


presumably ok for the lead acid start battery but not good for extended long life of the gel house batteries


fortunately the boat also has a swedish septor dual battery separator that electronically splits and controls house batteries independently of the start battery


this has a voltage trim pot which has allowed dropping the voltage coming from the alternator/start battery to a lower/healthier level for the gel house batteries


to keep an eye on it all i recently installed a small voltmeter and a couple of press switches just inside the companionway


mainly this is to tell me when the start battery voltage is dropping below the solar fed house batteries


then i pop across some little jumpers 

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I'm not sure how accurate the Voltage reading on instruments are, but I would expect OK. To be sure I'd check them with a good multimeter.


I'd just like to point out that the volts displayed on your instruments will be the voltage at their source of power - perhaps a distribution bus - this may not be the same as the batt voltage. It pays to check now and then that the voltage at your instrument or panel IS the same as at the battery terminals directly. Any discrepancy in voltage (normally lower!) is due to resistance in the cables and connections between the measurement point and the batts. This is usually corroded terminals in the joins between, but can be damaged or undersized wiring.


The MAXIMUM allowable voltage drop to any device being powered is 5% - so 0.6v on a 12v system. A lower voltage will draw more Amps, so minimising voltage drop is desirable.


It is really important for a battery meter to accurately measure the volts of the batt. 10ths of a volt (or even less) are very important, so the meters normally (as does the BEP one mentioned) have a calibration function, so you can set their display to match the ACTUAL voltage at the bats. If you do this then their calc of battery state of charge will be more accurate.

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IT re the AGM,

Not a big deal but my last 130 ah AGM only lasted a year and a half before it wouldn't power the instruments for more than 3 hours.


I'll be the first to admit it had a hard life being on a boat that was used for mostly racing at the time, by the end of the RNI it was stuffed.

After a bit of research popular opinion (not necessarily scientific fact) backed up what I was experiencing in that they don't cope with being severely deep cycled as well as good quality lead acid.

I know all batteries will die an early death with harsh treatment but I have had better performance out of car batteries than the AGM.

Li-ion seems to be the magic bullet once the price comes down lets hope so.

Supercapacitors anyone?

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1 year's pretty poor


what make AGM and how was it charged?


 The leading advantages are a charge that is up to five times faster than the flooded version, and the ability to deep cycle.


AGM offers a depth-of-discharge of 80 percent; the flooded, on the other hand, is specified at 50 percent DoD to attain the same cycle life. 


As with all gelled and sealed units, AGM batteries are sensitive to overcharging.

These batteries can be charged to 2.40V/cell (and higher) without problem; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages).


Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell), and a direct replacement with a sealed unit could spell trouble by exposing the battery to undue overcharge on a long drive.



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Very abusive :twisted:

Like I said it was flogged mercilessly, sometimes (often) when your tired whilst racing you forget to run the engine to charge the batteries. I won't name the manufacturer as it wasn't their fault but believe that lead acid is tougher when abused. 

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Another option for a battery monitor is this from Blue Sea


I selected it because it can monitor DC and AC with a single instrument (BEP's cannot do this)plus it has the tank gauge and other options.

I also have separate ammeters on the outputs of both alternators, very Handy to see what they are up to.

You can purchase NZ made ammeters and voltmeters from Carel-Electrade


They were OEM suppliers to BEP in the 80's and my original gauges function perfectly

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