Batteries have not required water for over a year....WHY?

[Dave S]

This is a first for me because on the other boats I have owned the batteries have always needed topping up about twice a year. But the battery water (acid) on these have stayed right at the bottom of the filler ring ever since new. We have used the boat at least once or twice a week year round and the batteries are also on the battery charger every time we leave the dock.

I am not complaining mind you, but I would like to understand why we haven't had to add any water to them. :smt017

 

[Red Stripe]

Dave -

I'm glad you raised this and will be interested in the responses. The dealer told me to check the batteries routinely and fill them once/month. 105 hours since new in April and my water jug is still sealed waiting to be used.

 

[Dave M.]

Dave, I would say it is because your charging equipment is well configured. Batteries don't just lose water for no reason. They lose water then you put more current into them than is needed to replenish the charge. The excess current goes to break down water into hydrogen and oxygen, which then leave the battery as gas. That is how you lose water.

If you have your battery charger set so that you don't 'float' the battery at too high a voltage, then you won't be dumping current into it that it does not need, and you will reduce the loss of water. For most batteries, 2.25 volts per cell is a good float voltage. That works out to be 13.5 volts. I was just looking at the spec of a ProMariner ProTech charger yesterday. It is set for 13.5 volts float, and 14.7 volts bulk charge. I personally think the 14.7 is a bit high, but the charger lets you set the time between 1 and 4 hours, and it would normally be set shorter on a small battery bank, say 1 hour. That is really not going to cause much water loss, the time is too short. And it does help carry out equalization.

Some ProMariner chargers have a float voltage of 13.3 volts. That is 2.22 volts per cell. Still OK, and will have less water loss. When you float the batteries this low, you do need to do equalization. But the chargers do that each time they are turned on. Also, your alternator will normally put out a higher voltage, which also helps

Equalization performs two totally different purposes. One is to reduce stratification in the cells and help knock off sulfation. The other is to bring any weaker cell back in line with the rest of them. The cells are never identical, there will always be a weakest one (it has lower specific gravity and therefore less charge), same as in a chain. If you apply a float voltage that is right for the battery set as a whole, the stronger cells will take more of the voltage, and the cells with less charge (lower specific gravity) will drop less voltage. That cell is therefore not getting the recommended 2.25 volts per cell, and therefore won't get charged totally. Since you can't apply voltage to a single cell to bring it up, you have to apply a higher than needed voltage to the whole set to force the weak cell to come up.

Oh oh, I think I wrote way more than you asked. :smt015

Bottom line, I think you just have chargers working fine, and just need to make sure the batteries get a dose of equalization now and then to prevent sulfation and keep them in line.

 

[Hampton]

Nice post, Dave, even if it was your 666th. Care to tag on any info on AGM's (Optima Deep Discharge) for the crowd?

 

[Dave S]

Dave

Thanks for the explanation. I probably should conclude that my GUEST battery charger is doing it's job. The one installed in my 260DA is a 20 Amp model and it goes thru three stages of charge if I understand the manual properly. One would be for fully discharged bateries where 10 amps of current is applied, Then it has a finishing stage where it holds the voltage to 14.3 and gradually reduces the amount of amps to the batteries, and then it goes into float mode while holding the charge at 13.3 volts.

Dave

 

[Dave M.]

John, I have no experience with the Optima batteries. I do have experience with AGM batteries in general, but that experience is 10 years old and all bad. The AGM batteries we had at work, generally 24 or 48 volt sets, ranging from 350 to about 1000 amp-hour, were the most maintenance intensive batteries I ever encountered.

Since that time I think a lot has been learned about how to make AGM batteries that don't dry out, don't overheat, don't have thermal runaway, and don't have very high internal resistance as they age. I have seen many flooded cell battery plants that were still good at 20 years old. I have never seen an AGM last past 10 years. But these are in situations very different than in a boat.

I'm not saying the Optima, or any other good current version AGM battery has these problems. I know that there have been ways found to get around some of these, such as the spiral winding.

I think the current AGM batteries are quite good. There are really only two issues that I know of. One, you really don't want to overcharge them. Secondly, and maybe more important, they need to stay cool. Temperatures over 100 F are very hard on an AGM battery. Hmm, I guess there is a third. You would be doing well to have your charger be one that has a remote temperature sensor. Place the temp sensor at the battery, so that the charger knows the temperature of the battery surrounding. Then the charger can adjust its float voltage in accordance with temperature. See the very bottom of this link for some data on battery life vs temperature. From Trojan, "6. Correct the charging voltage to compensate for temperatures above and below 80o F. (Add .028 volt per cell for every 10o below 80o F and subtract .028 volt per cell for every 10o above 80o F)"

Also note on the last link above from Trojan that they recommend a float voltage of 13.2 for flooded cells and 13.5 for AGM.

 

[Dave M.]

Dave, your battery probably does not have to be fully discharged for the charger to go into the 10A charge mode. All it has to do is to be able to accept 10 amps without the terminal voltage going over 14.3 volts.

So the logic something like this. Upon turn-on, apply 10 amps until the battery voltage reaches 14.3 volts. Then apply 14.3 volts until the current the battery draws tapers off to some preset value, or a specific time is reached. Then switch to float voltage of 13.3 V.

Not being fully charged is hard on a lead acid battery, so the charger should try and charge it as quickly as possible. Thus it will charge it with the full 10 amps until the battery voltage comes up to 14.3. It also means if you partially discharge your battery, you should recharge it at the first opportunity to maximize its life. Don't let it set partially discharged if you don't have to.