Exponential Performance of Batteries

[Just Plain Jeff]

From time to time, we all check the condition of our batteries; whether we have a meter, dial or LED monitor. What is not known quite as widely is that battery performance is exponential, which is just a fancy-dancy way of saying that a read-out of 6 volts does not mean that a 12V battery is half dead.

Here's a little chart to explain:

12.66V=100%
12.45V=75%
12.24V=50%
12.06V=25%
11.89V=Discharged

Here's another way of looking at it:

100%=1.265 Specific Gravity
75%=1.225 Specific Gravity
50%=1.19 Specific Gravity
25%=1.155 Specific Gravity

All of the above are calculated at 80F degrees or, 26.7C degrees.

Correction factors are significant, depending upon the ambient temperature of the location of the battery.

For example, if it is 90F, a correction factor of +4 is indicated.

If ambient temperature is 50F, a correction factor of -12 is indicated.

Although many of us may think that minor battery fluctuations are insignificant, they are not.

[Jon Wehrenberg]

To further the discussion for those who haven't had the pleasure of being driven crazy by battery problems, your gauges may show voltages of something like 13.75 volts. Maybe higher.

Those are charging voltages. On a typical 12 volt system it is not unusual to measure 14 or 14.25 volts. There is a range of voltages your battery sees especially when connected to more sophisticated charging systems. Think of charging voltages the same way you think of the pressures required to put 100 PSI of air in your tires. You could get your tires up to 100 PSI if the air hose had 100 PSI in it, but it would seem to take forever. So instead you use a hose with 125 PSI (or more) to fill the tires to 100 PSI. Same with your charging system, except there is a limit to input voltage so you do not damage the batteries.

The voltages Jeff posted are of a battery at rest, with no charging circuit or loads attached. And most importantly the voltages are after the surface charge has dissipated. That usually takes several hours.

If you have any questions about your batteries or battery charging system you will only need a cheap multi-tester, and information such as what Jeff posted.

To increase your battery life never ever let your batteries discharge deeply. If you start your generator and charge your batteries every time they drop to 12.5 volts your batteries will outlast those that have been allowed to routinely drop to 12 volts or less as some auto-start systems require.

[Joe Cannarozzi]

Our 86 has a Heart inverter, 2500, and the remote shows a light on for a-c input and it never goes off. Im assuming that this has something to do with why it wont work. It has a very good 4 stage charger built in and that does work. Im guessing that the automatic switch gear wont switch because of the a-c input, but Im lost past a general inspection. Also, generator auto start/stop is inop, I think. I say this because theres 12 new batteries, and I cant remember if they ever went down to 12 flat, I believe they were once and the auto start did nothing. We put a 1200w inverter,temp.,in the rear for essencials it cost only 100$ and runs the ice maker, fridge, entertainment center simultainiously and thats really all we need, as we always stop to eat. The issue that is also important is the switch gear in the original system, the convienyence of not switching the cords of what your powering to different plugs is almost an nessessity. Thought I saw someware new inverters that are not constent on but sence power draw and auto on and off. This heart is 21yrs old unless its something minor it will eventually get replaced. Anybody have any feedback?

[dalej]

Joe are you and Deb going to go the POGII? We can compare coaches there. It would be nice to look over your setup and see if there are some simple resolves, maybe your setup is just fine the way it is.

[Jon Wehrenberg]

Joe,

Be very careful before you use "dollars" as the criteria for inverter selection. The inverters used by converters are selected for their features, not the least of which is a 120V AC power supply that will actually provide your various devices with the type of power they require.

I would urge you to do some checking on the quality of the power supply because you have such diverse devices that will use 120V power such as computers, the TV, a refrigerator, and whatever you plug into an outlet.

The other consideration is the charger portion of the inverter needs to be such that its charging profile meets the requirements for the batteries you use now, and those you may use in the future. A typical inverter charger can be programmed for charging typical wet cell batteries, AGM, or gel cell, all of which have different charging regimen.

[Joe Cannarozzi]

Jon, Im currently not changing anything because of the charger and the fact that it is the correct component and it works well. I understand you get what you pay for but how could this temporary inverter damage things? We have used the same 1200w in previous campers and had flawless operation? Eventually I will upgrade to state of the art and when we do it will be everything all at once, a package of some sorts so for right now we are going to put as little as possible to these things. Well probably end up with something with significant solar capabilities. What about our auto start/stop for the generator how do you check and test this? And do you think that if I can eliminate the source for the false ac input light the inverter might go on? HEART interface 2400

[Jon Wehrenberg]

I'm not sure of all the systems out there and have never had to shop for an inverter so I am surely not qualified to recommend a specific product or talk about the virtues of one over another.

I have followed other discussions on other bus sites and find there are inverters and then there are inverters. Ditto with generators. The key to our comfort is having generators and inverters which supply clean power that does not adversely affect all of the devices we tend to use.

With respect to the self start, I do not have a clue as to how to test your specific one. What we have is one called "Watchdog" and it can auto-start due to low battery voltage or loss of shore power depending on how we set the switches. It also shuts the generator down if there is a cooling problem or oil pressure problem.

[dalej]

Jon can you give me the volts for the charging and the float rate, I have AGM 8D's (4) I put these batteries in back in 1999
and did not change the inverter settings, had acid batts before.

I was looking at my inverter and can set the volts of each.

[Jon Wehrenberg]

These are approximate, but I think a typical charging voltage is around 14.25 for lead / acid. AGM could be damaged at that level so I think the typical voltage there is about 13.8.

It has been a while since I reviewed my inverter book, but it goes into some detail. I think you might be able to do a web search also.

This site is a good starting point, and I think you can link through it to other sites to expand the information.

http://www.ccis.com/home/mnemeth/12volt/12volt.htm

Charging protocols are part of the inverters and on mine a set of dip switches on the back of the panel are the only things that have to be set for specific batteries. The rest of the regimen is taken care of from that point on so you don't need to know the specifics.

[dalej]

I just went out and looked, the charge rates lowest setting is 14.1 and float rates highest setting is 13.5. The batts are showing 13.5 with a volt meter in the float mode tonight.

[Jon Wehrenberg]

My gut tells me that is OK. Your inverter charger may have been set for AGM batteries.

In my bus I have a single alternator so both the wet cell bus batteries and the AGM house batteries are being charged simultaneously. Based on what I have seen on my voltmeters when running, my voltages are consistent with my understanding of the safe level for AGM. I am presuming that the bus batteries tolerate lower charging voltages.

The charging through your alternators is your other concern.

[dalej]

Yes that is why I will take my house alt. off and take it in to have it checked. I might just see about a new 180+ alt. I bet mine is just a small auto alt.

[Jon Wehrenberg]

Dale,

You don't need to remove an alternator to check its output.

Just go to fast idle and check the voltage at your respective battery posts. If the batteries were near fully charged when you checked you should see the absorbtion charge voltage or float rate.

If it is too high the voltage regulator can be adjusted......unless it is a small automotive type with the internal voltage regulator.