Battery System—  History and Information


The first batteries I was able to acquire for the solar system were four sealed lead acid AGM batteries rated at 100 amp hours. These had been used in the oil & gas fields near Dallas and were typically used to power radio telemetry equipment used for monitoring. In most cases the solar panels used to charge them had inexpensive regulators that were prone to failure in the field. While an analysis of these failures would make an interesting case study, I believe it is sufficient to say that the batteries were not treated very well.


Typically batteries used for radio telemetry are removed from service in the gas fields after a year to 18 months or when they will not hold a charge for two days of little or no sun. In most cases I have found that batteries get over charged and or are run down below their ability to recover.


The four that I started with were considered throw away as they would not hold a charge overnight or could not hold one for two days of no sunlight.  I found, through just experimenting, that I could bring them back to most of their capacity by slowly recharging them using both a bulk rate charger and a PWM charger.


Using an inexpensive car battery charger, purchased at Harbor Freight, I would start by bulk charging the battery to get it up to about 10.5 volts (measured using a fluke DVM) and then switch them to the PWM charger. The PWM charger I used is from Pulse Tech. It uses a proprietary algorithm and has, as part of the charging cycle, a battery test phase. At 10.5 VDC the charger registers a battery charge status indicator of 25%. If the voltage is below 10 VDC the charger will say it is a dead battery and not try to charge it. Therefore, as phase one of the recovery process I had to get the battery to a level where the PWM charger would see it.


NOTE: While other PWM charger are available—in my view Pulse Tech charger is the best one for recovering batteries under the conditions I have described


I would start the PWM charger (why PWM) and simply leave it on until the battery indicator showed 100%. This would typically take any where from 24 to 96 hours—sometimes longer. Often the battery would then self discharge and I would start the process again. In several cases I had to spend a week rejuvenating a battery but in each case I was able to turn a discarded battery into one that I considered fully functional for my purposes. Clearly this is not practical in the commercial world but for the amateur, it is a great way to get free or almost free batteries that can be recycled.


One of the reasons I believe I was successful was that I carefully monitored the battery voltage while it was under charge. I consulted the manufacturers data sheet and found that I needed to stop the charging when the batteries reached 14.4  to 14.7 VDC.  In two cases I had to run the cycle twice but eventually the batteries all responded and I felt like I had recovered them. I also kept track of the battery case temperature and did not exceed the maximum case temperature. As I was working with low charge currents, typically 2 to 4 amps, it was not difficult.


Although I never did a real load test on the rejuvenated batteries, I was able to use them in my solar system and they have worked well for the last six months. While they hold a charge and work well for my daily requirements, the real measurements will come when I finish the monitoring system and have data to quote.


I will quickly admit that the process is not really scientific and that several of my conclusions are empirical in nature. However, I take the position that I got the batteries for free and was able to make something from them. In addition, I followed the manufacturers charging instructions and kept the discharge cycle within their specifications.


The point I wish to make is that anyone who is going to use batteries should know what the manufacturer has rated them for. Specifically, how to charge them and how to keep them working by discharging them correctly. Translated, it is necessary to have a system battery charger that will be tailored to the battery type used in the system. My plan is to build a battery charger that is tailored for the batteries I will use in my system. Although the chargers used during the recovery process did a good job, they are not designed to be used as the system charger.


Battery sources:


For anyone who will take the time to look, I am confident a source of used batteries can be found. In my case there are numerous oil field service companies and oil production companies in Texas that use batteries in their monitoring and telemetry systems. These are typically replaced after two years or less and are frequently abused during their life. These may not make the ideal battery bank but if you have enough of them, you can get the capacity necessary to put a small solar system together. I have also found that either the service companies pay to have the batteries hauled away for scrap or get $15.00 to $20.00 per battery for their lead content. Either way, cheap or free batteries are available if you look around and ask for them.


Chargers used in the recovery:

The bulk charger I use was purchased for $22.00

(on sale at Harbor Freight). It is not anything special

and is a simple car battery charger. The PWM charger

was purchased on-line from Pulse Tech.  While this

company is not well known, their products are great

in my view. 



Battery technology:


While I do not plan to write a discourse on battery technology, I would advise anyone undertaking a project using batteries to become conversant in the types of batteries they have access to and more specifically what is the charge rate for that type of battery. Each type is different and may vary from manufacturer to manufacturer. This last point is of paramount important because improper charging causes the majority of battery failures. In my case it was clear from distorted cases that several of my batteries had been badly over charged and that the resultant heat had distorted the cases.  There are links at the end of this page that should be a starting place for any serious battery work. If the reader is like me, building a specific AC or grid powered bulk and PWM battery charger is not difficult. If you want to purchase one, this can quickly become a quest as most battery chargers are not well documented and more often then not, do not work well with multiple battery types. Therefore, do not accept a blanket statement that a charger will work with AGM batteries or any other type you may have. Find out what the charge rate is and then check to see if it will work with the batteries you have.


Be sure to stay within the per cell charge limits that are specified by the battery manufacturer and use a DVM that has been calibrated. I use a Fluke 23 and will recommend them. I will not recommend cheap DVM’s made in China as I have found them to be far less accurate.


First battery system:


My first four batteries were AGM sealed lead acid batteries from Power Sonic. These were rated at 100 amp hours and had a discharge rate, at about 16 DC amps, of five hours. Therefore, the theoretical limit of my four batteries is 20 hours at my present discharge rate of about 12 amp hours. If I recharge during the 20 hour discharge duty cycle I can extend the useful discharge time accordingly.


The present system has two duty cycle times; the morning for about 2 hours and the evening for about 4 hours. In amp hours this would be a total draw of 12 amps X 6 hours or 72 amp hours of draw. If I have good sun throughout the day I get about 3.5 amps for about 2 hours and then I get 6.5 to 7.5  amps for about 4 to 5 hours and then I get about 2 amps for another 2 hours. Bottom line, I charge about 36 to 46  amp hours and draw about 72. Therefore, with my present solar charging capacity, I have to recharge the batteries from the grid every two days to keep even.


Most of these charge cycle times are observations during the day when I manually record data. The times are an approximation but clearly show the need for a monitoring system that will automate the grid charging cycle as required and keep track of the actual amp hours of charge I get from the solar panels. Having useful data on the charge rates and consumption rates is, in my opinion, critical to making a system that is both practical and meets the requirements placed on it.


Using a monitoring system will allow me to reposition the solar panels and use the grid charger during the evening when the electricity rates are at their lowest. I can also check the output from each solar panel to graph individual performance.


Second battery system:


I have just acquired another six batteries that are AGM batteries from MK Battery (these were throw away so I consider my effort as recycling). These are rated at 86.4 amp hours each. I have to recover them in a similar fashion to my original batteries. This will take, if history is a good predictor, about 4 to 5 days for each battery. In the end I will not really have recovered them to 100% but I will have a useful bank of batteries I can use for the system that should have (I am guessing here) 85% to 90% of their original factory capacity.


As a starting point the batteries were tested before attempting the recovery process:


Battery #          Initial voltage

1                      12.65

2                      12.34

3                      12.55

4                      1.278

5                      1.192

6                      12.51

As can be noted, two of the batteries were almost flat and I questioned the possibility of recovering them. However, after three days, battery #4 has reached a voltage of 12.56 VDC and continues to take a charge from the PWM charger. To get it to the starting voltage of 10.5VDC, I used the bulk charger and then switched to the PWM charger. After 6 days on the PWM charger, the battery recovered and will be allowed to sit for three days to see if it holds a charger (battery #4 voltage was at 12.85 VDC after 12 hours of sitting ). In the interim, I will start on the recovery of the next battery.


One problem that I will eventually face is that the maximum charge these new batteries can take is 14.1 VDC or 2.30 volts per cell. For now this does not cause a problem but it does mean that I will need a separate AC charging system for the new batteries because the minimum charge rate for the Power Sonic batteries is 14.4 VDC. Fortunately my solar charger has a switch that I can control, using a simple relay, to adjust the charge rate for each battery. To start with, once I have recovered them I will set them up as a separate battery bank and simply switch between banks. Discharging one while I charge the other. The monitoring system will keep track of the rate and adjust the solar charger accordingly.


The complete restoration process will be documented here but it is expected to take three to four weeks to recover them completely and longer if it does not go well. (free does not always mean no work involved)


Once they are recovered I face the problem of charging them with the present solar system. As noted above it is clear that there is insufficient charging capacity using only the solar panels and as I have almost no wind at this time of the year, the wind turbine is almost useless.


I have two solutions that I will work with. The first is a VAWT Vertical axis wind turbine that is being fabricated and will move to the testing phase in about two weeks. The second is to replace the Morningstar solar regulator with an MPPT controller as this has a theoretical improvement of 30% in charging current from the present solar system. While a test will be necessary, the hope would be that I can get 9 hours (during the summer) of charge time at 8 to 10 amps. This is at the maximum edge of the MPPT controllers specifications.


The two wind turbines will not really offer much in the way of charge current until the fall when the leaves are off the trees. Then I expect to see a combined output, on a windy day, of around 12 to 14 amps. However, as the sun will be lower in the sky, I will see a reduction in the solar output. I will also be able to document how effective wind turbines are when used in an urban environment.


Additional comments on using 12 Volts and not shifting to 24 volts:


The original configuration used 12 volts due in large measure to the cost of the inverters. Twelve volt, modified sine wave inverter are relatively inexpensive and will satisfy my AC power requirements. If I shift to 24 volts the cost of the inverters goes up dramatically. Therefore, while it is not as efficient as using 24 volts, the 12 volt system is cheaper to start with. Unfortunately one of the down sides is that my choice of commercial high current 12 volt solar panels is limited. I would like to purchase panels that put our at least 10 amps but they are not available in 12 volts.


                    Battery recovery process:


Number of days on Charger                        Final battery voltage                 Nr. of days it held a



Battery 1      6 days                            13.1 VDC                               4

Battery 2      4 days                            13.2 VDC                               4

Battery 3      1 day                             13.1 VDC                               4

Battery 4      4 days                            12.8 VDC                               3

Battery 5      8.5 days                         13.1 VDC                               4

Battery 6      4 days                            12.77 VDC                             4



The voltages shown were taken when I disconnected the charger. After several hours they settled to 12.7 VDC to 12.65VDC. This would show a 100% state of charge but it does not translated to a battery that is 100% recovered.


As can be quickly noted I have been at this recycling and recovery for about 4 weeks and have been able to recover all six batteries.  Battery #5, was in the worst shape of all of them and had the oldest installation date. While it did recover, it took the longest.


The lesson is “never give up”.


As I said earlier, just because batteries are free does not translate to no effort to make something useful out of them. With the effort I put in I now have another 518 amp hours of batteries to add to my present 400 amp hours. (this is not precisely true as I really have no idea of the actual capacity of the recovered batteries. I know that it is not 100% but I do not know how much less. The numbers quoted above are the values of the batteries in new condition)


Now to wait four days for the next six batteries and then to have battery cables made to interconnect the new battery bank. I will use 4 gauge wire to do the interconnections and will purchase commercial cables. The cables used in the previous battery bank came from the junk box. The supplier for the battery cables is local to me but he will take orders over the phone.


The recovery of the next six batteries is the next installment.



Note on Pulse Tech:

These chargers are available on the web from several distributors. The charger has made all the difference in recovering otherwise dead batteries so I consider the cost well worth it.

Note on PWM:


The link is to the Morningstar site but other sources also testify to the benefits of using a PWM battery charger. I suggest a bit of research if there is a doubt as to using such a charger.

October 27, 2008


Final installment on the last six batteries:



As can be noted, the batteries were in sad shape. Two showed signs of over heating and one had a case that was cracked on the top. 


I started the recovery process as before by using the bulk charger to bring each battery up to about 10 VDC and then switching to the PWM charger. The first day of the recovery process, 8/21/08, started with the recovery of battery 7. As it was over 12 VDC I put the PWM charger on it and started battery 8 on the bulk charger.  By 9/15/08 I had completed all 6 batteries and all were at 100%. I have added these to my battery bank using my home brew battery cables.  This brings my battery capacity to about 1,420 amp hours.



Before closing this section I feel it is important to point out that with a large battery system, such as I now have, it is paramount that safety, when working around the batteries, be a first concern. If this battery bank were to be shorted it will result in a fire and possibly an explosion. Be very careful when working around high amperage DC as there is always a possibility of a short that could result in a fatality.  Just because it is 12 volts does not mean it can not kill you if you do something foolish or irresponsible.

Herb & Barbara our interests and family

Battery Nr. number

Initial voltage