How to test sealed 12 volt Gel Cells

Score one "good answer" for you elnav. Have been wondering about that for ages, but never got around to asking. I use 12v Gel for my fish finder.

Cheers

Thanks! Gel batteries are finding more and more applications. So are AGM batteries. The totally sealed nature of them allow for shipping as regular goods instead of "hazardous" goods. Both have better shelf life than flooded wet cell lead acid.

Unfortunately, many of the applications find their way into seasonal or infrequent use applications. Most people do not bother to maintain these batteries in a proper charge /float setup and the lead acid batteries get sulfated as they self discharge. The ubiqutous power cube seldom provide optimum charging voltages. Depending on line voltage they may either over charge or under charge the batteries. I have one flashlight that is rechargable. Sure enough when I needed it the light failed within 15 minutes. Opening up the light revealed a 6V lead acid Gel battery. Didn't take long to determine it was sulfated. A day or so of "treatment" and now the battery will accept a charge again. But the power transformer provided with the light fails to bring it to full charge. Solution! Use a 5 watt solar panel supposedly intended for 12V use. It was a perfect match and I got 220 milliamps of charge current. Battery voltage at the terminals never exceeded the rating printed on the side of the battery. Mfg recommended charge voltage was 7.25V and so far I have not seen more than 7.1 V from the solar panel.

Laughing Jaguar hasn't yet responded with data on how big a batery bank he uses or how much curent is needed for the sideral drive on his telescope. I'm guessing he is using batteries of a similar type to what I have been experimenting with.

Laughing Jaguar wrote:

2. Do a voltage drop test while putting the battery under a heavy load.

REPLY the traditional "toaster" type test may have been adequate for conventional starter batteries found in cars. However the Gel batteries now being used in many applications from children's toys to serious field equipment used by professionals requires a much better test procedure.

One test method finding increasing favour is a capacitance type test. In North America a company by the name of Midtronics has seen their products used in everything from production line testing, to auto dealer service departments. Unfortunately the method by which the tester is connected and the nature of accessories that may be connected to the battery at the same time has a big influence on test results. A recent test of factory production run batteries using a Midtronics tester, revealed plus /minus 20% results. The discrepancy was determined to be caused by test lead connection issues. As a result, dealers for this particular brand was advised to use a more standard test lead connection using test terminals bolted to the battery. The batteries in question have recessed screw terminals so a brass fitting fastened with a provided cap screw is used. In addition the battery terminals should have all oxide cleaned from the mating surfaces. Because the test is capacitive in nature physical size and ohmic resistance contact of the test leads come into play.

A more reliable method is also more time consuming. It requires the use of a small to medium size inverter and a timer driven by the inverter. These inverters have a low battery shut-off that cuts out at 10.5V Measuring the time interval from start of test to end as indicated by getting a measured voltage of 10.5V

Loading can be adjusted by using a multi step resistive heater or a purpose built test fixture. The trick is to adjust the discharge current to a level consistent with the size of the battery. A 10 amp discharge may be suitable for a car battery but inappropriate for a small 4 amp hour portable battery pack.

Because my own inverter puts out a modified sine wave, not a puer sine wave I chose to use a small clock motor drive for my timer. Digital clocks will miscount the zero crossing transitions on MSW waveforms. The battery council can provide data on the correct test procedure for a given battery type and size. To time duration of test th eclock is driven by the output of the inverter. When inverter quits th eclock stops.

Typically; capacity is measured as a 10 hour or 20 hour curve or sometimes on a 100 hour curve. This is the time it takes for the available charge to be totally used.

As a rough and ready test I often just use a 10 amp draw and measure the number of hours it takes to discharge. This works fine for automotive and golf cart traction type batteries. For smaller portable instrument batteries a 1 amp discharge rate is more appropriate. If size of battery is less than 4 A-H capacity I suggest using something even smaller like maybe in the range of 100 - 500 milliamps.

My reason for using the inverter instead of a direct resistive load is that the inveter design provides for a constant current drain despite changes in battery voltage as the battery discharges. The swith mode design of the inverter compensates for the decreasing voltage of the battery. A resistor across the battery would result in a decreasing current draw as the voltage drops.

Lead acid batteries display specific typical voltages against specific loads. There used to be charts for that and that is what I was looking for when I came across this web site which if you scroll down has lots of specific information for specific lead acid batteries.

http://www.necon.co.za/megahow.htm#hown

I suspect there is enough information in these charts to tell you battery condition and charge although I do not see the volts vs. load chart.

I'll leave you with this and go back and see if I can find the load chart. There should be no need to discharge a battery to determine its condition. You should be able to momentarily put a specific load across a battery, read its voltage, and know the state of charge.

The old automotive battery load tester does that but in a very crude way not suitable for the types of batteries you are talking about.

j.

j.

If you add a small correction to that for temperature, I expect that would be accurate enough for driving a small boat around with an electric motor.......It basically what I have always used and it worked well enough - at least it will keep your "rowing" to a minimum for the boat owner!!

For the telescope, it should be enough too, but invest in a small charger to allow the batteries to be topped up from the car you use to drive to where you are going to view from, or even a long 12 volt cable from car to the scope!

Jack wrote: I suspect there is enough information in these charts to tell you battery condition and charge although I do not see the volts vs. load chart.

REPLY: Those voltage versus state of charge tables are predicated on the batteries not having any sulfation. In other words conditions in a perfect world. Its fine for brand new batteries.

Experience has proven that batteries left in a partially discharged condition will end up with a difficult to remove coating of lead sulfate on the plate surfaces. This effectively reduces the total capacity of the battery. The remaining battery plates will charge and you will read a voltage corresponding to state of charge.

Unfortunately that does nothing to indicate how much capacity is effectively available in that battery.

This is why the voltage indicator method has lost credibility as an accurate indicator. It is also one of the reasons companies like Midtronics did the research to develop a better test tool than the "toaster" load resistor.

I recall one time we were working on some USCG patrol boats. They were complaining that brand new batteries would not deliver enough power. The load resistor test still gave a "good" indication but the battery would not support inverter operation for more than 10 minutes. Normally a brand new battery would sustain the inverter load for an hour. We eventually figured out that the battery in question had a two year old date code and the battery had sat uncharged on a store shelf for 12 months, then another six months at the station supply room, before it was finally placed into service. 18 months of storage without a maintenance charge had allowed the formation of lead sulfate which had then hardened to the point it could not be reversed with a normal charge. The voltage reading was good.

All the battery manufacturers will tell you that specific gravity reading is a better indicator of state of charge than voltage. However, you cannot use this test with sealed batteries. This was the second reason for Midtronics developing their test gear.

The original post seemed to indicate the use of sealed batteries. Which is why I suggested doing a complete discharge cycle to determine the actual as opposed to claimed rating for the battery. That would give a bench mark against which the battery could now be measured using the Link 10 or similar batery meter.

When I worked for Xantrex I was asked to look into this entire technology with a view to possibly licencing the technology or perhaps developing our own for our chargers and battery meters. Just so you know why I have all this information.

I've just returned from the Adirondack mountains and four consecutive nights of observing. The battery pack did fine, providing ample power.

There was no reason why it shouldn't. Temperatures never dropped below 60° F at night and the only drain were the stepper motors in the tracking system. I never had to turn on the dew heaters to keep the mirrors clear.

The real challenge is in the winter when I am wearing an electrically heated suit that draws about 10 amps.

The equipment discussed here is outside of my budget and I can not justify the costs when there are higher priorities to deal with.

To help eliminate the uncertainty, I will simply make it a point to

1. date stamp each battery when it's first placed into service

2. maintain a ready state with a float type charging system.

3. peridoically, test it by putting a known but modest load across the terminals for a period of time and track the voltage.

If a chart of these test is kept, any changes in the battery's performance should be predictors of impending failure.

I've learned a lot here. Thanks to all for the time spent responding to this and the effort you made to educate me.

L. J.

I don`t know what temperatures you will experience, but having the electric suit seems a bit OTT.

Why not buy clothing that protects you fully, but does not need electricity?

If you do have a battery failure, will you die of cold before getting to your car again?

L.J. you haven't indicated the size of your sealed battery pack.

For the telescope drive I don't expect your draw will be much. However the heated suit is going to eat lots of amps. 10 amps load current is going to kill a full sized car battery in one night session. How do you plan to recharge?

I had assumed you would be on a budget, ( who isn't) but my suggestions are all intended for a low cost budget. Have a look at eBay for Link 10 products since this is now a discontinued model and therefore will be dumped by dealers with excess stock. My little 300 watt inverter was $39.95 (full retail price) and these can often be picked up at garage sales and flea markets for $5 - $10. The timer is a motor driven alarm clock with arms on it instead of a digital readout. Cost $0.50 at a flea market. The load device is a 100 watt bulb. It sounds like you already have a multi meter if you plan to log voltage readings on a regular basis. I think most of these have a 10 A or 20A scale built in. You can fabricate a makeshift shunt from a piece of copper flat stock or even a piece of heavy wire. Accuracy will be within 5% as long as the "bus bar" stays close to room temperature.

Email me PM if you have questions on how to set this up.

With 3 digit display and using the lowest scale you are reading voltage to millivolt resolution. Close enough for what you are doing.

I have salvaged literally thousands of dollars worth of batteries using my Pulsetech device. People tend to dump a sulfated battery when it doesn't take a charge or hold a charge long enough. I get them for free. A week of treatment is usually enough to bring them back to life even if they have been sitting unused for a year.

In the North East you should be able to get one for around $80. Or order direct from the factory in Texas. - www.pulsetech.com

The suggestion of charging the sealed batteries from your car is valid, as long as you do not try to pull too much current through the cigarette lighter plug. These are often fused at 8 Amps and on some cars I have found the DC socket wired into the same circuit as other things like fuel door release, or window motors or some other accessory. If you are going to charge a bigger size battery ( say more than 50 amp hour capacity) from the engine alternator use booster cables. Keep the engine at a really fast idle of 1200 RPM or so.

Good luck and hope you have great observations.

elnav