What voltage do you let your batteries drop to? Or Smartgauge percentage?

[Norman_E]

I have a Smartgauge. After two days on a mooring running the fridge it shows the house batteries down to 69% charged and at 12.35 volts. The batteries are sealed "maintenance free" lead acid. How low is safe, before I need to run the engine and re-charge them?

 

[jiris]

There is no a "short answer". It all depends on the type of batteries (are yours starting or deep cycle ones?), the number of cycles you want to get out of them, the rate at what they being discharged, temperature and some other factors. Also the numbers from different sources may vary. Two rules a are standing out:
The less you discharge them before recharging, the longer they last. The lifespan start decreasing rapidly below 50%.
12V is quite drastic and should be avoided, below 11.6 the battery can be considered as permanently damaged. But as I said, these numbers may vary. Your 12.35 V can be considered pretty normal and I wouldn't worry about it. You have to balance the cost of a larger battery bank and the cost of earlier replacing a smaller one. What you have is probably somewhere around that point.
If you want more accurate numbers, there is whole host of sources you can google - but in the real life it is all very "rubbery" and your own results may vary wildly from those you find...

 

[RichardS]

I run my batteries down to 12V but that's with the fridge/freezer running which means the rested voltage would be a couple of points higher.

The old ones lasted 11 years which seems pretty decent and we installed new ones yesterday. Bearing in mind the problem we had in swapping over three 60kg dead weights, I'm hoping that the new ones will also last a similar time. :ambivalence:

These were all heavy duty deep discharge AGM batteries so probably more voltage tolerant than those referred to by Jiris.

Richard

 

[PaulRainbow]

The OP states his batteries are "sealed "maintenance free" lead acid". Not AGM and not deep cycle, so with respect to Richard, his figures will result in very short lived SLA batteries.

I would try and keep them no lower than 70% discharged, so his figure of 12.35 volts is about right, the batteries won't be rested at that reading, so will actually be a little less than 70% discharged, especially if the fridge or other heavy loads are on. With minimal load on the batteries, i'd be putting some charge back into them at 12.35v. Perhaps a solar panel would be a good idea ?

 

[ip485]

I agree with PaulRainbow; this could be a very easy way of destroying the bank pretty quickly. Solar is a very good idea as well as obvioulsy running the engine in neutral while at anchor if there is no other alternative.

AGM are far more resilient, and I have personally found that Lifeline AGM are even better - but at a cost. Even then I think running the bank to below 50% other than perhaps once in a while should be avoided.

Do also check the your SmartGauge is really telling the truth and that when you charge the bank you really are charging to 100% (or as near as you can get, the capacity will decline over time). The SmartGauge will gradually drift over time, so it may tell you that the bank is at 100%, when it is not. Repeatedly charging to less than full will also do a great deal of damage over time, so really should be avoided.

Finally, it is also very important to make sure your charger is set up correctly so as to be compatible with the batteries. Each type of battery has its own charge profile. A mismatch may well mean the batteries are not being fully charged ever.

A trick can be to charge the bank to an indicated full, turn off the charger, and then after a short while back on again. It will not go to float immediately and you can monitor how long before it does. The longer it takes the more chance the batteries are not being as fully charged as you might think.

PaulRainbow is the expert and I have put this in slightly laymans terms. It is a very involved subject if you wish to be technical and an especially important area for true liveaboards spending lots of time off the "grid", but probably too much for one post and in answer to your question.

From experience, I hate anything to do with batteries! Bl**dy daft technology, and, there is no doubt hopefully LiFePO4 are the solution especially when the price is closer to AGMs.

 

[wizard]

With the smart gauge I consider the floor at 80%. Longer battery life is important due to the cost and aggro of replacing them.

 

[ip485]

Wizard - that does seem very conservative, but I can fully understand your concern! For me, I think the issue is that at anchor that is possibly very little power unless the bank is very large indeed and with some of us using more power hungry devices may be a challenge, but obvioulsy it depends very much on the individual and your needs. I think PaulRainbow is about right and 70% would be fine, especially if you arent cycling to this level on a regular basis. I think again this depends on your use. If it is the typical few weeks a year crusing with the rest of the time back on the charger every night clearly it is going to have far less consequence than night after night on the hook. It is all about cycle times and the manufacturer should give some guidance on this aspect in their technical literature.

The other point that I think is relevant and perhaps not commonly appreciated is that I think for most batteries it will help to charge as quickly as you can. We have had a discussion before about using multiple chargers and whether or not and when one will go into float, but, that aside, when the battery is "low" it will absorb rapidly and that is usually thought to be a very good thing to help reduce the rate at which the plates sulphate.

 

[boatmike]

I agree that wizard is conservative at 80% but more to the point I think the way smartgauge calculates percentage can be misleading. Far better to monitor voltage. The original OP says his smartgauge indicates 69% at 12.35V . With brand new batteries I get a similar reading. However the programme within the smartgauge seems to "learn" the maximum charge that the batteries will take and gives you a percentage of that and with old batteries you will get a different result. So by all means watch the percentage as a guide but use the voltage as a value to make decisions by. My own thinking is that with good deep cycle batteries 12.35V is quite acceptable. I do agree that at below 12V however you will shorten the life if you don't recharge.

 

[lw395]

With the smart gauge I consider the floor at 80%. Longer battery life is important due to the cost and aggro of replacing them.

But that way, you are carting around twice as many batteries as you need at great cost.

The optimum answer probably varies according to how you use your boat. A liveaboard doing 365 cycles a year will have a different answer to someone using their boat at weekends, only a few of which are heavy on the batteries.

Also think about the value you get from using your batteries.
If you end up eating in the pub to avoid running your batteries down below some arbitrary optimum, you may find it's an expensive way of saving money.

Battery manufacturers publish cycle life vs discharge % graphs, then you have to take into account the effects of age.
There's probably no point trying to make your batteries last more than say 7 years.

A big part of the question is 'when will they next get charged?'

 

[GHA]

, before I need to run the engine and re-charge them?

That's probably the more important bit to look at, takes ages to get lead acid back to really fully charged - very unlikely just under engine. And must be the no1 way batteries get killed, general consensus seems to be best to get really 100% about once a week or less.

You need some solar

 

[Poey50]

That's probably the more important bit to look at, takes ages to get lead acid back to really fully charged - very unlikely just under engine. And must be the no1 way batteries get killed, general consensus seems to be best to get really 100% about once a week or less.

You need some solar

Smartguage seems good at percentage discharge of capacity but is over-optimistic on giving a measure of 100% charge. We use the fridge as a nice-to-have and will use it if we can be sure of getting the batteries up to full charge - which either means an overnight on the mains charger within a week or leaving the boat on a mooring with 100% showing on the Smartguage and with solar to bring it genuinely fully up to charge. In three years, the batteries have never been less than 80% discharged and usually not less than 85%. I know this is conservative but we lived without a fridge on a boat for 15 years without distress and I like the idea of getting my money's-worth from the batteries.

We also have a Victron BMV 700 which, because it is user-programmable, gives a better reading of full charge than the Smartguage (which, in turn, seems more reliable for percentage discharged).

 

[ip485]

Strangely, in this magical world of battery science I have read those who say that frequent but small use of the batteries without ever a reasonable discharge can be equally damaging. In other words running down occasionally to say 80%, but at all other times on the grid. I think the logic is that the charger rarely comes out of float, and when it does, its is running for a very short time on bulk. I have done several complete cycles on my bank from around 50% charge to as close as to full as possible. At the lower end of charge I can put around 54 Amps into the battery running two large chargers simultaneoulsy - but it is interesting the amount this backs off as the batteries come up to "full" charge, and also how little charge the batteries will then start to accept. At the top end, this means very little charge is going into the batteries, and I guess the argument is this is exactly the area in which they are likely to sulphate, and without ever being cycled, the sulphation gradually increases. On this point I am not sure whether the science really supports the argument or it is just another myth and the extent to which it depends on the construction of the battery.

Also, I think it is well rehearsed that bring the batteries up from 50% to say 80% is relatively quick and easy - the problem with any lead acid battery is that the last 20% to 10% is really hard as the battery accepts less and less of the charge. I can run my bank down to say 55% overnight, and either on solar by before lunch they will be back to 80%, or, on the Genset possibly a couple of hours, but the last 20% will take until the evening, which is a really pain. It isnt shortage of power, but the inability of the batteries to absorb the available power.

Perhaps, if there were room, for the serious liveboard, and in an ideal world, you would have two banks so one could be on charge, while the other was in service!

 

[prv]

For what it's worth, and with no particular science behind it, my answer to the OP's question is 50% on the Smartgauge. I rarely see that, but if I do I will start thinking of motoring (or motor-sailing) for the batteries' benefit where I might otherwise sail.

Otherwise, I don't really make any decisions (engine on, marina vs anchor, etc) with the batteries in mind, and my normal pattern of use keeps them reasonably topped up.

Pete

 

[Norman_E]

Paul Rainbow's reply and others have reassured me. I did see 67% discharged and 2.55 volts before re-connecting to mains and turning on the charger. The reason the voltage was higher is that I now have 4 x 50w flexible solar panels on the bimini and between writing my first post this morning and arriving at a jetty with mains power this afternoon the sun had been providing enough power to balance the fridge load. I hardly ran the engine so the alternator has contributed very little. It now appears to me that I can run the fridge for two days or slightly more "off grid" without doing damage to the batteries, which is the result I wanted from adding the solar panels. Where I am now there is no ice available, but it seems clear that if I can start the fridge off with a couple of ice blocks I can stay off grid and still have cold drinks for about a week even in the 30+ temperatures here, and aim to keep the Smartgauge percentage at 60% or higher.

EDIT. The batteries are fairly new, bought in May this year and are 3x110AH plus 1x110AH starter battery. When I recharge on the mains charger I keep it on until it goes first into float mode and the turns itself off. Its a 30A pro Charge Ultra, and on connecting today it charged for a time at just over 30 amps, is now down to 16 amps and will gradually drop its output until it is doing no more than balancing the 12 volt load from lights, fridge, and this computer.