Wednesday, December 26, 2012

Gel vs AGM Deep-cycle Durability

Selecting Sealed Lead Acid Batteries for Deep-Cycle Durability

Sealed Lead Acid (SLA) batteries offer several advantages over conventional wet or flooded acid batteries. Primary advantages include lower self-discharge rates, higher charging rates, better recovery from deep discharge and no exposure to acid vapors or liquid. There are two types of SLA batteries, Gel and Absorbed Glass Mat (AGM), each with their fans and detractors. There is conflicting information, with groups sources claiming that one battery type or the other is best. It is hard to get a clear understanding of which is the superior solution based on information available on the internet.

Below are some of the things I learned using (in many ways, abusing) deep-cycle AGM and Gel batteries under very high charge and discharge use on an extended daily basis everyday for years. I found several points of conventional wisdom to be flat out wrong.

I designed and installed three vehicle based battery bank systems. These were installed in Sprinter vans equipped with full dark-rooms including  film processors and air-conditioners. The film processors had a peak power draw of 1500W. The vans used Xantrex sinewave inverters, chargers and inverter/chargers along with gel (East Penn in Deka or MK markings) and Lifeline AGM batteries. There were multiple occasions where employee's drained the banks until the inverter shut down due to under-voltage and they were often drained to 10% remaining capacity.

The battery demand in the film vans was heavy, reaching as high as 150A draw during use of the film processor and charging at rates up to 100A during driving. Total usage per stop average around 40 amp-hours and totaled 350-450 Ahrs per day. Both the gel and AGM setups were recharged from Bosch 150A alternators during driving. Charge rates observed for the Gels was 20-30% of bank capacity (up to 110A from the alternator to the 360Ahr gel bank). Charge rate for the AGM was slightly higher 20-36% of bank capacity (up to 120A into the 315Ahr AGM bank). Typically the charging currents would charge at approximately the same current as the level of discharge. That is, a bank that was at -80 Ahr would charge at around 80 amps and drift down as the battery bank was charged. This was similar between both the AGM and Gel banks with AGM's drawing slightly more current from the alternator.

The Battery Banks
The first van was initially fitted with two 6v/180 Ahr gels to make a 12V/180 Ahr pack. These were charged with a TrueCharge 40 and a WestMarine battery combiner that connected to the alternator via  6 ga cable to the vehicle battery. Ultimately the 6 gauge cable and wiring resistance between the alternator and the starting battery prevented effective charging from the alternator. These batteries we driven below 20% capacity on a regular basis. We added two more 6V within a year for a total capacity of 12V/360 Ahrs. Note that 6V batteries usually have heavier plates which should allow more life-cycles from the battery.

One set of four 6V gels was wired with 0 gauge welding cable, the other set was done with custom bent bus-bars Here you can see the battery pair on the left already has the 6V terminals tied together. I actually tied all four of the 6V terminals (the pair with the cable and 2nd pair directly to their right in this image) together to balance the load and charging between the four batteries.

Here is the partially wired system. I tried to keep the wiring as short as possible and use bus-bar where possible. For this install I used a Prosine 1800 inverter with a TrueCharge 40 battery charger. You can see the welding cable coming from the dual fuse holder on the left to the Blue Sea Systems ACR (Automatic Combiner Relay) battery combiner. The 500 A shunt in the center of the image goes to the Xantrex Link 10 battery monitor installed in the cab to allow the driver to monitor battery state and charging level while the engine is running.

The system control panel is mounted in a bin attached to the passenger seat. The Link 10 battery monitor and ProSine control panel provide the x-ray tech full access to the system status and control of the inverter/charger. The Link 10 also offers a serial port for logging system status, allowing analysis of usage patterns and charging rates during the day.

Here you can clearly see the positive and negative welding cables feeding through the floor at the front left of the battery tray. Note the two orange connectors. In the event of an issue with the battery power system plugging the orange connector together will bypass the system and run the truck directly from shore power. It is a good idea to always have a backup plan.

On this bank the positive  power cable goes from the large blue fuse block on the right side of the panel and connects to a fused cable coming from the alternator output stud. The negative cable runs between the alternator mounting stud and the battery bank current monitoring shunt. Note the heavy duty threaded rode securing the battery bank. The rod goes all the way through the floor and is backed by washers and nuts under the vehicle. Batteries are heavy and must be secured well enough to stay secured during a collision. 

The second power system used three Lifeline (AGM) 12v/105 Ahr batteries (315 Ahrs total) . Later we added a 4th Lifeline (420Ahrs total). I tried the Lifelines based on good reviews on a couple of marine sites and the vertical terminal bolts lent themselves to the easy use of bus-bars. I very much wanted to be pleased with the Lifelines.

The Lifeline three battery bank with bus-bars. The copper bars were coated with clear polyurethane to reduce tarnishing with the coating sanded off at contact areas. Just barley visible at the bottom edge between the batteries are the spacers used to prevent weight shift during driving from placing stress on the terminals. Batteries are heavy.

Here is the Lifeline battery bank and the Prosine 2.0 (a combined 2kW sinewave inverter, 100A charger and auto load balancing transfer switch) installed in the vehicle. The feed through holes for the welding cable was moved to the rear of the battery pack and the single inverter/charger allowed a smaller footprint for the electronics board in this installation. The photo was taken during the install before the final wiring cleanup. Again, note the heavy retaining threaded rod running through the floor to hold the batteries securely.

The Lifelines appeared to loose capacity much quicker than I expect. I realize that comparing 6V deep-cycle Gels to Lifeline AGM's in this environment may favor the gels (6V batteries typically have improved durability due to thicker plates) but I am confident the results were so convincing that this difference could not be the only factor. Every few weeks the Lifelines were topped off with a AGM specific charge profile that included an over-charge and they were Equalized when recommended by Lifeline tech support. In the end, the Gel batteries survived much better under the abusive deep-discharge conditions seen in our application.

We also tried Lifelines in a digital van with much lower power requirements to see how they do under less extreme loads. Here system components are placed in the minivan rear seat well to test for fit and layout optimization.

In this digital van the battery system is covered by an access panel attached to the original rear seat hinges. This allows maintenance access while protecting the battery and electronics. The Lifelines did not perform much better in this application and were eventually replaced with a single 12V gel battery.

The Results
When new, both the gels and Lifelines charged fast from the alternator, stood up to heavy loads and performed well. But under extended heavy use the East Pen gel batteries outperformed the Lifeline AGM batteries by a considerable margin. In fact, all of the Lifeline batteries were removed from service within 24 months. Lifeline offered to prorate a discount on replacement batteries but offered no assurance that the new ones would last any longer. By comparison, the first set of gel batteries was still going strong after over 6 years of service when the van was removed from service due to an engine failure. I estimate that the two film vans with gel battery systems saw over 50k hours of use between 2003 to 2011. The systems were all converted to the lower electrical demand digital systems starting in 2011.

I feel that the system designs were sound, we had battery monitors on both the gel and Lifeline systems and used the recommended charge profile for the Lifelines in the Prosine 2.0. We would even do an equalize charge on the Lifelines whenever we noticed a performance decrease. I feel that we made a fair comparison between the Lifeline and East Penn gels. Ironically, gels are often maligned for their "sensitivecharging profile and described in many documents as not appropriate for direct charging from an alternator in marine or RV applications. Many also claim that gels cannot charge as quickly at AGM batteries or that applying high charge rates will damage the gels. Keep the alternator voltage under 14.05V and just let the gels charge as fast as they want. 

Secret to Great Charging
The real secret to great charging was low impedance wiring between the alternator and battery bank. The systems used two 0-gauge welding cables, one from the alternator output positive post and the second between the alternator mounting bolt and the negative connection (current shunt stud) at the battery bank. Of course there were fuses at both ends of the positive line (since there was a power source at both ends of the cable) and a combining relay was used to only connect the battery bank to the alternator when the engine was running. The combining relay protects you from running down your starting battery. The vehicles were also plugged up when the batteries did not fully charge from driving but they often went weeks without being plugged up. 

But as they say, there is no such thing as a free lunch. Charging the battery banks at these levels places a high demand on the alternator and combining relays. The alternators (Bosch 150A) lasted three to five years with the gels but much less with the AGMs before they began to be unable able to maintain 14V into a significantly discharged battery bank. The AGM cooked one alternator withing a year but it may have only been a 120A unit rather than the standard 150A used on the Gel installs. The AGM bank was replaced in the vehicle at around one year of service with the alternator being replaced sometime in the next few months. I suspect the alternator had been stressed by the AGM bank.

We tried several brands of alternator and alternator regulators during our early experiments in search of high charge rates. Bosch was the clear alternator quality leader. Bosch alternators automatically adjust their output voltage for temperature so the charging voltage was higher when it was very cold and lower when it was very hot. This also allowed the alternator to protect itself by lowering the output voltage when the alternator began to over-heat. I believe the Gel's worked better at this as the charge rate seems to stay higher on the AGM bank when the alternator lowered the output voltage to keep the alternator from overheating. I suspect this accounts in part for the shorter alternator life seen on the AGM bank.

One install used a Stancor Type 120 power relay (100 A continuous, 400 A inrush) to combine the starting and house battery bank at key-on. After the 2nd replacement failed it was replaced with at Blue Seas Systems combining relay. We never had a single failure of either of the Blue Sea Systems combining relays. I have been thoroughly pleased with every piece of Blue Sea Systems hardware we used.

I hope this information is useful and can help you get the best performance from your battery bank. Please feel free to contact me if you have any questions or comments.