Jeff,

Sam Allen

Sam,

Your question is actually more complex than you probably thought since there are several different charging issues involved. If you read the fine print on any new deep-discharge battery, it will give its warranted life in a fixed number of total discharges. In other words, a battery with a 6 year warranty may only last 2 years if deeply discharged more times in the shorter period than warranted. This means a battery that is discharged 25% every day then recharged will normally last much longer than the same battery that gets discharged 75% one day each week. However, on the other side of this debate, a deeply discharged battery can take a huge charging rate which charges much faster if using a generator or solar array without out-gassing which will increase the need to add more distilled water more often.

The other re-charging issue relates to a large battery capacity matched to a small solar array. Typically what happens is the first day the battery starts out at 100% full, then is discharged 20%. The next day the solar brings it back to 90%. Again, the next day the battery is discharged another 20% so it now is at 70%, and the days solar adds another 10% leaving it to start the next day at 80%. This cycle continues until the battery keeps cycling in the 30 to 50% charged state and never gets fully recharged. This type situation will damage the battery because when the lead plates are never brought back to 100%, the surface area will soon develop a layer of calcium or other minerals which become insulators between the acid and the lead which reduces the plate area exposed to the acid and this means battery capacity is reduced. In many cases, this damage cannot be reversed.

The real answer to your question is based on how the battery is being re-charged. If you have a quality 3-stage battery charger that will “back-off” the charge rate as the battery reaches full charge to avoid gassing, then you should get the best battery life by shallower daily discharge and re-charge cycling regardless of battery capacity.

Hope this helps,

Jeff Yago

Share this entry

Jeff,

I have a deep cycle battery which we use with an inverter at craft shows to power the credit card machine and sometimes lights or fans. After using it all day and before going to the next show, I would like to be able to check the battery level.

How can I check the battery level?

I plan to get a second battery as a backup since we do multiple day shows and can’t afford to lose function of the credit card machine, but it would be good to know how much drain has occurred after a day’s use.

Thanks for your help.

Sam Allen

Sam,

Knowing how much amp-hour charge is remaining in any deep cycle battery has been a problem still waiting for a solution, as there is no perfect meter that can tell you this. The only way to know the exact charge level of a lead-acid battery is to insert a calibrated hydrometer with temperature correction into each cell. Obviously that’s not going to happen, so you could take a voltage reading with a digital volt meter.

The problem is the volt meter will not give a true voltage reading while the battery is under load, as it will read too low, but it will also read too high when there is no load and the battery is at rest or was just charged. Many companies make a battery amp-hour meter that keeps track of how may amp-hours you put into the battery when charging, then will subtract from this total during discharge, but this also has to keep correcting itself due to temperature changes and how the “rate” of discharge can affect this reading and these meters cost several hundred dollars.

You did not say what inverter you are using, but many will include a battery volt meter which will give you a rough idea of battery charge.

Here is my suggestion. Buy a volt-meter from Radio Shack for about $15 that provides a digital display. Set for the lowest DC voltage range it has that can easily read in the 8 to15 volt range, but not more than 50 volts.

Add a “small” load to the battery. Nothing large and do not connect the inverter. Maybe a small 12 DC light or small DC fan you indicated you had, just enough to pull off the surface charge but not enough to start drawing down the voltage.

Note the voltage reading and compare with the following:

20 to 30% charged – – – – 11.60 volts

30 to 50% charged – – – – 11.90 volts

50 to 60% charged – – – – 12.20 volts

60 to 70% charged – – – 12.45 volts

80 to 90% charged – – – – 12.50 volts

Over 90% charged – – – 12.66 volts

The above chart is just a starting point for you. Once you take a few voltage readings and then see how long your equipment will run, you will be able to customize this chart for your specific battery. Please also note this chart is based on your battery being at 77 degrees F. If the actual battery temperature is higher or lower, this voltages will shift some, but not major until it is over 90 degrees or below 50 degrees.

Good Luck,

Jeff Yago

Share this entry

I hope this is a easy question.

I have a motor home that has 3 deep cycle 12 volt batteries.

I want to keep them charged with solar as much as possible.

Will connecting 2 or more 45 watt separate systems charge them better than just one?

As they are all the same watt I am not sure adding 2 or more same watt systems will charge any more than 1.

Help Please.

Thank you

Att: Help Please:

I assume you mean connecting each battery to its own solar module and charger verses connecting one solar charging system to all three batteries. For good battery voltage balance, its better if you connect one solar charger to all batteries wired in parallel. I don’t know how you could do this separately unless each battery can be disconnected from the others, since normally they all are wired to the same 12 volt positive and negative mains. Assuming you could separate them from each other, as soon as you connected a battery that was at a higher charge than the others, current will flow from one to the other until they are equal in voltage.

Keep it simple,

Jeff Yago

Share this entry

I have a solar system (4-100 watt panels, voltage regulator, 6-6volt deep cycle batteries wired in series and parallel ( a battery “pack”), a low power and high power (3000/6000 watt Vector Inverter with soft start technology)).

This system is at my cabin which I use only 10-15 times/year for 4 days at the most per visit.

I use the low power (400 watt inverter) for lights, radio, a small TV and other miscellaneous needs and don’t use much power from the low power inverter. In fact the blower internal to this inverter rarely comes on and I have never had a problem getting enough power out of the battery “pack” to supply my low power needs.

I use the high power inverter for my well pump only which draws high current ( i.e. 60 amps a.c for about 1 second) at start-up and then for 2-3 minutes of a 12 amp a.c. current draw, till the pressure tank is full. I estimate that I start the well pump anywhere from 3-30 times per visit ( over a period that might range from 2-4 days and thus often get a solar charge during this time).

Lately the battery “pack” will support the high current draw at 12 volts (measures above 13 volts at these attempts) without dropping below the 10.5 volt minimum required by the inverter for only 1-2 times to start the well pump. After 1-2 times of starting the well pump the battery pack voltage drops to about 12.4 volts and when I attempt to start the well pump, the voltage drops below the 10.5 volts required by the inverter and the inverter goes into a fault mode. The batteries seem to have enough capacity right after they have experienced a charge cycle from the solar panels and the voltage from the combined 6-6volt battery “pack” above 13 volts.

I suspect the batteries need to be replaced even though the specific gravity of all cells in all of the batteries measures above 1.270. The batteries are about 5.5 years old.

Considering my application, where I don’t use the batteries frequently and probably don’t run them down to less than 60-70 % of full charge, what type of batteries would you recommend? Deep cycle, marine, automobile? 6 or 12 volt? Any other recommendations?

Thanks for any advice you can give me.

Dave Keese

Dave:

I suspect what you have is a battery bank that has out-lived its usefulness. When batteries get old or have had too many long periods of low charge, they develop a “false” charge. The old plates become covered with calcium buildup which insulates the lead from the acid and reduces the charge capacity of the battery which is like making it smaller. With no load connected to batteries in this condition, the meter can read a fairly high voltage and make you think the battery is fully charged. This same problem can also make the solar charge controller think the batteries are full and will stop the solar charging. However, as soon as you connect a large load the voltage will drop in only a few minutes to the level of a discharged battery due to the limited capacity of the built-up plates.

Taking a voltage reading of a battery that is not supplying power and one that is under heavy load will both give you a false voltage reading.

The more accurate voltage reading is when the battery is under a small load. Normally a specific gravity reading is more accurate, but you would need to check every cell as one or more could be really low and causing the rest of the cells to never reach full charge.

The old lead-acid deep cycle battery is still the lowest cost battery you can buy (although all battery prices have recently increased about 40% due to world-wide lead shortages) and they are very forgiving of over-charging, which only causes some water loss. Since your cabin is empty for many days each month and the system is not providing any power, you may want to consider a deep-cycle AGM sealed battery. However, remember a sealed battery must be charged with the correct charger settings or you can boil out the moisture and it cannot be replaced.

Time to buy batteries!

Jeff Yago

Jeff,

Thanks for the response. Since you didn’t really indicate what type of batteries (deep cycle, deep cycle marine, or automobile) I suspect you are advising deep cycle even though I have a very high current demand for a short period and probably don’t discharge the batteries below 75-80% each time I go to my cabin. I also assume there is no advantage in using 6-6 vol batteries or 6-12 volt batteries to form my 12 volt battery bank. Am I correct in these assumptions?

Thanks again,

Dave Keese

Dave:

I would only use deep cycle batteries and yes, there is a big difference between using 6 volt and 12 volt batteries. Almost all deep cycle 6-volt batteries like golf cart and L-16 sweeper batteries have double the thickness in their plates since the 6 volt battery only has 3 cells yet takes up the same physical base size as a 6 cell 12 volt battery.

Most “deep cycle” 12 volt batteries are usually the RV/marine type, which are much heavier than a car battery which is why they are called “deep cycle”, but they are about half the capacity and life of a golf cart or L-16 “deep cycle” industrial battery. I would not use the lighter weight 12 volt batteries in this application, and if you need a sealed battery, the 6-volt golf cart battery is also available in a sealed version.

Good Luck,

Jeff Yago

Share this entry

Jeff,

Bill:

Sounds like you have been trying some interesting things with your vehicle.

For most charging applications using non-sealed lead-acid type deep-cycle batteries, most battery and charger manufacturers recommend a maximum charge rate of C/5 for a battery almost discharged until it reaches about 80 to 85% charged, then back down to a C/10 rate. This means you take the total amp-hour rating of the battery, say 220 amp-hours (typical golf cart battery) and divide by 5 giving 44 amps for a maximum charge rate to start with. This limit is an “average” and takes into account how a battery will get very hot if charged too fast, while trying to reduce the re-charge time as much as possible. Since your RV type battery will have a lower C value, its charge rate will be lower. Of course if you are using a trickle-type charge with a very low charging rate, it will never be able to reach these maximum charging rates for either battery type.

Hope this helps,

Jeff Yago

Share this entry

Jeff:

Bassam:

A 5 watt solar panel puts out about 0.3 amps of current. A typical deep cycle battery could have hundreds of amps capacity, and needs a charging voltage much higher than the battery voltage or you cannot “push” the charging current into the battery.

Your solar charger needs to be operating above 14 volts when charging to reach the higher charge level, and I bet it is dropping down to around 12.5 volts as soon as it is connected to the battery.

A 5 watt solar module is almost a “toy” when wired to a deep-cycle battery. You need a solar module in the 20 to 40 watt range to fully charge this size battery.

Good Luck,

Jeff Yago

Share this entry

Jeff:

Jimmy,

Based on your battery description and age, this is a Model #PVX-6220 Concorde “Sun-Xtender” VRLA sealed Battery. This is a “maintenance-free” absorbed-glass mat (AGM) battery, which means the acid is absorbed and cannot be re-filled.

Any quality lead-acid battery, sealed or open, will have several things that will affect its life. If it was maintained near a full charge most of the time, it should last 6 to 8 years. If it was located in an area that did not get too hot or too cold, it will last longer. If its normal charge and discharge cycle did not cause the depth of discharge to exceed 20% (80% charge remaining), it will last longer. If it regularly was discharged down to a total discharge state, or was left totally discharged for months, most deep discharge batteries will fail years before their advertised life.

If your batteries meet any of these conditions, I would say they are worthless by now. In addition, the “maintenance-free” sealed batteries are very sensitive to charging voltage since a lower quality charger that does not measure battery temperature can re-charge to quickly and over-heat them. Since they are sealed, this can cause a venting of gasses through the over-pressure safety valve and this loss of moisture cannot be re-filled since they are sealed. Sealed batteries usually dry-out and fail much faster than open type lead acid batteries (batteries with fill caps), even if they were properly maintained.

I suggest taking these to a battery re-cycler who may pay you something for the scrap lead which has really going up in price nationally in the past few months. In fact, battery prices have increased over 50% in many cases due to the high overseas demand for scrap lead.

Good luck!

Jeff Yago

Share this entry