Shore Power and Batteries
In an insomnia moment last night, I got thinking (scares me too ;) ). If one were to believe Nigel Calder you get maximum use from batteries if they are allowed to discharge to near 50% before recharging. Am I prematurely killing my batteries through my habit of plugging my boat into shorepower and therefore the battery charger whenever she is in her slip? I tend to use my boat close to every weekend for daysails, so in reality she is on shorepower about 90-95% of the available hours each week, I would estimate that I am only discharging my batteries by 5% or so when she is out of the slip.
Yes, you're technically wasting a lot of the potential power from your batteries, but I wouldn't worry about it. :) If all you use is 5%, then all you use is 5%. It is more the people that bring their batteries down 90% that need to worry. :)
The bigger question is whether you actually need to keep the boat charger powered 24/7 - if you have refrigeration, obviously the answer is yes so why worry about it (rhetorical question). If not, I would tend to believe Calder rather than any suppositions you receive here.
You post a deceptively simple question. The answer isn't simple at all.
Assuming you have flooded lead-acid batteries aboard, there are several factors to consider.
Lead-acid batteries deteriorate when allowed to sit without a charge on them. This is true even for new batteries on the dealer's shelves; they begin to deteriorate the moment they leave the factory. Nigel told me he's measured differences in new battery capacity as much as 30%!!
There are several factors involved in this deterioration, including corrosion of plates, sulfation of plates, stratification, contamination, and others. Of these, the most important contributor to reduced battery capacity is generally caused by sulfation, i.e., the formation of PbSO4 crystals which attach to the plates and, if left in place, eventually embed themselves into the plates and reduce capacity.
Lead-acid, flooded batteries have a significant self-discharge rate. That is, even when just sitting, they lose charge rather quickly. As they lose charge, the rate of sulfation increases.
Now comes the kicker: you're damned if you do and damned if you don't. Even a flooded lead-acid battery which is left on float voltage of about 13.2V (for a 12-volt battery) will lose capacity over time.
That's why it's recommended to periodically "exercise" your batteries and to "equalize" them at intervals of several months. Among other things, exercising and equalizing tends to reduce the effects of stratification and of sulfation.
A cruising boat which sits at the dock for several weeks or months and then goes cruising will normally see an alarmingly low voltage after being on batteries alone at anchor for awhile. However, after several discharge-charge cycles, voltage will tend to hold in there longer and the batteries will appear to have increased their capacity to handle normal loads.
So, what do you do to improve battery longevity?
As best I'm able to discern, through formal testing and thru personal experience, for boats at dockside much/most of the time I believe it's best to:
1. use a smart battery charger only, and preferably one which uses pulse width modulation (PWM) technology...like the Iotas;
2. leave the charger on all the time (assuming that you have reliable dock power and that you have a properly installed AC system and battery charger);
3. equalize the batteries at intervals recommended by the manufacturer. Note that the PWM chargers like the Iota MAY reduce the need for equalization;
4. exercise the batteries periodically, drawing them down to not more than 50% of capacity and then fully recharging them;
5. use ONLY distilled water to be sure the electrolyte is at least 3/8" above the plates;
6. use Hydrocaps or WaterMiser caps to reduce the need for adding water;
7. keep all connections very clean and tight;
8. be extremely careful not to introduce contaminants into the battery cells; and
9. monitor battery voltage periodically, with and without charging and load.
Sound like a lot of work? Not really, just a little attention on a regular basis.
A good list Bill. :D
Thanks for the responses
Yes they are wet cells,
1. On the Christmas List, currently is a Newmar RM 2020
2. Currently am doing this reliable shore power but see above for charger.
3. I need to add this to my routine maintenance lists, I meant too when I read it in Calder's book but forgot (gotta stop drinking rum while reading :D )
4. On the list to be done biannually, Should I change this to quarterly or annually?
5. I do
6. Haven't bought yet, but I check my batteries voltage and water level everytime I get to the boat (weekly give or take)
8. Try my best
9. See #6, part of my going sailing routine.
Thanks again for the response,
the RM2020 seems to be discontinued by Newmar. Make sure you get one with an EQ option...and I would do it quarterly and AFTER you take it down to 50% and the do a full 3 cycle charge.
Bill's list is great! Bill...how would you modify for those with AGM's? I would take your list and :
1. NOT do #2...but simply top up and re-top every few weeks up to a couple of months max.
2. Be sure all regualtors and chargers are set properly for the voltage differences needed from factory "wet" defaults.
3. Skip #'s 3, 5 and 6.
4. Intentionally run down the batts to 50% at LEAST once a month and provide a 100% top up.
Any other thoughts?
Good...seems like you're on the right track. I'd strongly suggest that you look at the Iota chargers. IMHO, these are the best on the market in terms of both value for the dollar and in terms of their ability to keep your batteries happy over the long term. They come in a variety of sizes from 10A to 90A, and are surprisingly economical. Look on eBay. Be sure to get one with the IQ-4 smart charge option.
AGMs are very much like flooded cells, with a couple of differences which you've noted. You don't have to add water, they can be charged much faster, they have a lower self-discharge rate, and they may not require equalization as often as flooded cells. Charging voltages are very similar to those of flooded batteries.
AGMs can sulfate like other lead-acid types, and can therefore benefit from occasional equalization. Lifeline AGMs call for 15.5 volts for 8 hours, I believe; others suggest slightly different treatment, while some manufacturers say don't equalize at all.
One thing not mentioned above is the effect of temperature, especially on charging regimes. Unless you use a PWM charger like the Iotas, it's a good idea to have a battery temperature sensing charger which can compensate for wide fluctuations in ambient temperature. Problem is, recently we've seen lots of variation in the accuracy of temperature sensors...to the point where one doesn't know what to trust.
If the sensor reads higher than actual, your charging may be cut back to the point where the battery is chronically undercharged. Conversely, if it reads lower than actual, you risk overcharging and, in extreme conditions, worse.
For me, that makes an even stronger case for the Iota's and similar PWM chargers which don't require any special setting for battery type or temperature sensor, and which by design are highly unlikely to cause your battery to overheat.
Michael isn't the only one who has scary "insomnia moments". The more you think you know about battery chemistry, construction and behavior under varying conditions, the more you're prone to such occurences :-)
One more question
Is this what I'm looking for?
eBay Motors: Iota DLS-30 Battery Charger, 30A 12V (item 280283015710 end time Dec-06-08 07:18:51 PST)
Seems like a heck of a deal compared to the xantrex
Yes, exactly. That's the 30A model with internal IQ4 smart regulator.
They are incredible deals, indeed. I have two of them (a 45A model in my ham shack and a 55A model aboard my 42' sloop) and can vouch for their specifications. They are truly one of the few bargains in marine electronics these days.
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