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Old 13-08-2012, 17:12   #1
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When to Charge--Volts vs Amps?

While talking to another cruiser(not a wharf rat), we got into a discussion about when he charges his batteries while out cruising. He told me that he never believes the amount of negative amps his meter tells him as the day goes by. He only looks at the volt level.
With 12.7 a fully charged battery, he turns off every thing on his boat and waits 30 minutes and looks at the volt reading on his meter. If it reads 12 volts, he starts up his generator and recharges.
When my amp meter tells me it's at about -90 hours, I typically start up my generator. I only have 330 amp hours in my banks. I never looked at the volt level of the batteries. When I charge them, I know I won't ever get back to 0 amp hours(unless I'm tied up to a marina). Running my generator till I achieve 0 is a huge waist of time and gasoline. My friend waits till his amp meter says it's only putting out 15 amps and then he shuts down his generator because running it any longer is a waste of gas versus what he will get amp wise for his battery.
While I think my fellow cruiser has a good idea on when to charge his batteries, I feel that letting it get down to 12 volts is pushing the limits of draining a battery till it's totally drained(never a good thing to do).
Is my fellow cruiser on the right path as to when to charge the batteries? Throw out my amp meter and only go with what my volt meter says? Is a battery level of 12 volts going too low and damaging my batteries?
Hey, battery stuff can be a form of the black arts and I could use the input of other cruisers out there that have their own opinions.
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Old 13-08-2012, 18:10   #2
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Re: When to Charge--Volts vs Amps?

What he;s doing is keeping the bank at between 50% and 80% SOC. This is a common way to charge banks when cruising since the last 20% of charge takes forever due to battery acceptance. Using voltage alone is an old school way of doing it, that still works if you know what you're doing.

If you have better, trustworthy instrumentation, why not use it?
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Old 13-08-2012, 18:52   #3
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Re: When to Charge--Volts vs Amps?

Most voltage gauges are a bit inaccurate. Since I don't have anything other than a voltage meter, I start charging around 12.2 volts on the assumption that it's 'about' 50% or higher... I think if you get down to 12.0 your past 50%

http://www.windsun.com/Batteries/Bat...ttery Voltages
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Old 13-08-2012, 19:48   #4
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Re: When to Charge--Volts vs Amps?

If you have a cummulative reading amp meter like the Xantrex Link 10 or equivaltent (and it seems like you do) then that is a much more accurate basis for recharging than reading voltage.

When you shut off all loads the voltage can slowly rise for a long time, even 24 hours to reach a true resting state. But if you have 330 amp hours of battery capacity and starting from a fully charged state you have used 90 amp hours then you know that you have used 27% of the available charge. In which case I would wait until I have used another 90 amphours or so before recharging.

As the other poster noted, a good strategy is to cycle between about 50% and 80-85% of full charge. But always charge to 100% every few weeks to avoid long term sulfation.

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Old 13-08-2012, 20:06   #5
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Re: When to Charge--Volts vs Amps?

I normally just used voltage as my basis.... even though on two boats I had the latest (at the time) "battery brain". The bottom line cruising away from the marina seemed to be just charging when convenient. (ie: "going into Port O Spain today, that will probably be all day ... better get 45 mins charging in before we leave") or ("..oh looky here, batts are down to 11.6 volts.. arghh.. .. gotta get charged NOW!") If you are willing to let your fancy meter tell you what to do while cruising, then by all means use it! Otherwise, charge when you can, and when your batteries go bad... just replace them. With the above attitude I've never had them last less than 5 years.. and some Deka's I got with one boat lasted nearly 5 years after I bought the boat with them used... Most the batteries I've seen go bad prematurely are from lack of use... not deep cycling... Uh-OH... it's gonna be another Anchoring/Cat vs mono day!
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Old 13-08-2012, 20:57   #6
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Re: When to Charge--Volts vs Amps?

your friend has it right, well, almost. i also use a digital voltmeter to tell me the charge level in my battery bank. but i start recharging when the meter reads 12.4, which corresponds to about a 60% charge if memory serves me (will have to check the chart). then, like him, i run my charger until it starts cutting back below 10 amps (it's a 35 amp charger), figuring that it's not worth the extra gasoline to bring it all the way up. been doing this for eight years now and my golf cart batteries have lasted five years. i use a regular digital vom meter. recently i found a panel mount digital voltmeter on line for under ten bucks; i installed it and it gives me the exact same reading as my vom meter.

people will tell you that you must get one of those fancy amp hour meters but they also tell you that you need radar in the bahamas.

WHEN you check your voltage is important because the batteries have to have a 'rest' period to settle down to a steady voltage. i do this in the evenings, shutting down all my electric draws (mainly the fridge) and inputs (solar panel) for a few hours. then take the reading and turn everything back on.

keep it simple....
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Old 13-08-2012, 21:09   #7
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Re: When to Charge--Volts vs Amps?

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Originally Posted by onestepcsy37 View Post
i use a regular digital vom meter. recently i found a panel mount digital voltmeter on line for under ten bucks; i installed it and it gives me the exact same reading as my vom meter.
Hello onestep. I'm interested in your digital meter as I'd love to have a remote down below. It's not a necessity but at less than $10, well, I confess I'm intrigued. Where'd you find it? Details please.
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Old 13-08-2012, 21:15   #8
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Re: When to Charge--Volts vs Amps?

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Hello onestep. I'm interested in your digital meter as I'd love to have a remote down below. It's not a necessity but at less than $10, well, I confess I'm intrigued. Where'd you find it? Details please.
I have one of these 12v socket meters that works quite well. Between this, the solar panel display, the 110v charger, and even my mulitmeter, I get, at most, a 1v difference in readings between them all.
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Old 13-08-2012, 21:20   #9
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Re: When to Charge--Volts vs Amps?

When you get to know your batteries, you can figure out how much the voltage goes down under load. For example, 12.0v under a 10 amp load might be an equivalent SOC to 12.2v with no load after a few hours recovery. Once you do, you can decide when to kick on the charger with a quick glance at volts and amps.
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Old 13-08-2012, 21:42   #10
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Re: When to Charge--Volts vs Amps?

Thanks callmecrazy... I did a bit of poking around and found this one: Amazon.com: New 2 Wire Blue DC 4.5-30V LED Panel digital display Voltage Meter Voltmeter By Buyincoins: Home Improvement (already ordered)

For four bucks... well, I couldn't resist.
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Old 13-08-2012, 22:12   #11
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Re: When to Charge--Volts vs Amps?

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Originally Posted by janice142 View Post
Thanks callmecrazy... I did a bit of poking around and found this one: Amazon.com: New 2 Wire Blue DC 4.5-30V LED Panel digital display Voltage Meter Voltmeter By Buyincoins: Home Improvement (already ordered)

For four bucks... well, I couldn't resist.
I'm sure it will work. Reading voltage is pretty simple in terms of elecronics. I only got the equus because it's a fairly well known brand. And I think I got it on ebay for less than $10, after doing the research on amazon of course (gotta love user reviews!). $3.99 for a voltmeter sounds about right Just test it against a multimeter so you'll know how much the reading is off. It's usually not much, but a few tenths can mean a lot... As long as you know how much it's off (it should be consistent), then you can just take that into account when looking at it.
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Old 13-08-2012, 22:15   #12
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Re: When to Charge--Volts vs Amps?

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Originally Posted by callmecrazy View Post
I have one of these 12v socket meters that works quite well. Between this, the solar panel display, the 110v charger, and even my mulitmeter, I get, at most, a 1v difference in readings between them all.
BTW: I meant .1 difference....
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Old 14-08-2012, 00:15   #13
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Re: When to Charge--Volts vs Amps?

Too true (re the amount off) as I know at my pilothouse VHF the multimeter reads .1 less than at the batteries. [The solar regulator does give a matching number to the house bank.]

As I'll be ordering more solar later this week I'm hoping the fanatical battery watch will be a thing of the past. We do keep the start battery fully charged but I'm not quite clear on when I should start the engine to recharge the house... or for that matter, at what voltage I should stop the extraneous power usage.

Generally I have no qualms at 12.4, but 12.3? What is your cut-off point for "unnecessary" power usage? Here, 12.2 equates stop and most of the time the voltage goes up to at least 12.3 or 12.4.
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Old 14-08-2012, 00:34   #14
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Re: When to Charge--Volts vs Amps?

Is a SG test more accurate way to determine charge or more to tell cell to cell condition??

I realise that SG is not practical as a daily check.

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Old 14-08-2012, 08:04   #15
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Originally Posted by callmecrazy

I have one of these 12v socket meters that works quite well. Between this, the solar panel display, the 110v charger, and even my mulitmeter, I get, at most, a 1v difference in readings between them all.
1 volt is a huge difference. I hope you meant 0.10v, because the difference between 12.7v and 11.7v is huge when we're talking about SOC.
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