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Which are the "best" AGM/GEL batteries? A brief analysis.


meyerwh
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I am not an expert in the field and just like to share the info and ask for opinions and views on the problem. I do not consider Li batteries here. I have a Kodak 3kW 24V inverter, and 24V Li batteries are very expensive (about R17,000-20,000 for BlueNova 26V-77-2k and R17,000-22,000 for Pylon UP2500 2.84kWh). Second generation 25.6V Li batteries by e.g. Revov are currently sold with a minimum capacity of 5.2kW and go for more than R30,000.

The information in this document is collected and compared with a focus on a low-consumption off-grid scenario. Solar panels (1,320W) supply enough power during the (sunny) day to run all appliances with a minimum energy contribution by the batteries (maybe max. 10% capacity). During the night (14 hours without energy supply by solar panels), a fridge is a major consumer and I like to cater for about 1.2 kWh during this time.

I consulted the spec sheets of AGM or GEL batteries advertised (but some not available anymore as I found out) in South Africa. I compared the data on constant power discharge down to a cell voltage of 1.8V. I adjusted the rating of a battery for a 10h constant current discharge to 1.8V per cell (10.8V per battery) since some battery model names refer their capacity to a 20h discharge and/or to a discharge down to a cell voltage of 1.75V (10.5V per battery). As a second adjustment, I multiplied with a factor to “simulate” a 200Ah battery. I assumed that with this approach a common base is created for a better comparison.

The second info I compared was the cycle life at different depth of discharge (DOD). It refers to the number of discharging/charging cycles until the battery drops to a capacity of 60% of its original rating. These discharging and charging cycles are mostly laboratory experiments under idealised conditions. I focused more on the ratings up to a maximum of 30% discharge per cycle. To strike a balance between my battery size requirements and acceptable daily discharge, I am considering a battery bank of a minimum of 4.8kWh (4x 12V 100Ah or 2x 12V 200Ah) which would give me 25% DOD at an overnight consumption of 1.2kWh. For a 20% DOD, one would need a capacity of 6kWh which can only achieved by combining four 150Ah or six 100Ah batteries since one or multiple sets of two batteries have to be connected in series to obtain 24V for the inverter.

For the two “simulated” 200Ah batteries with a total number of 12 cells and a DOD of 25%, one would require for a constant power discharge of 85.7W per hour over 14h (1200Wh) a discharge per cell of 7.1W per hour for 100% DOD and a possible discharge of 28.6W per hour for 25% DOD. So, the value of 28.6 at a discharge time of 14h is considered here as the minimum requirement per cell of the two simulated 200Ah batteries. Squeezing the power consumption of 1200Wh into 10h would increase the value to 40W per cell at a time of 10h (which is 4.8kW energy). These values are only indications of what kind of performance figures to expect.

The constant power discharge graphs are shown in Fig. 1 and the cycle life graphs in Fig. 2. Green circles indicate acceptable performance.

775391070_batterycomparisonFig1top.thumb.jpg.16962e40e291d9d82bad00021f218426.jpg

1139712929_batterycomparisonFig1bottom.thumb.jpg.d2d801dcc73490d9a0b592d7bf07bb9e.jpg

Fig. 1: constant power discharge for “simulated” 12V 200Ah batteries given in W per cell

1577518563_batterycomparisonFig2.thumb.jpg.f7842591bef76a4b144c330d728972cc.jpg

Fig. 2: cycle life of batteries according to spec sheets or website information, 60% capacity as end-of-life is not always explicitly mentioned in the source

There are a number of other battery companies which I did not include here. Some perform too bad to be considered and others seem to be a rebranded product of another manufacturer.

According to one supplier, batteries based on lead crystal technology are not available in the country. The formerly supplied batteries by Betta Batteries are manufactured by GreenRhino, an American company. The Dutch office confirmed via email to me that they do not have an agent in South Africa. One can order from them and a 12-GRGS-200 battery costs 409 excl. shipping.

The Narada ICS range seems very good. In South Africa I could only find the NDF-Acme range which does not perform well enough and cost about R7,500 for a 12V 200Ah battery. I could not find a supplier for the lead-carbon battery Ritar DC12-100C. So, the good performers Narada ICS, Betta Lead Crystal and Ritar Lead-Carbon can be excluded since one cannot purchase them in South Africa.

The capacity of the Trojan SAGM 12 205 is only 174Ah which requires four batteries for the given scenario. One battery cost more than R8,000. Therefore, this battery can be ruled out.

Concerning the Allgrand batteries, the data are inconsistent. There seem to be a CNF and a CNFJ range and some spec sheets available from South African supplier cannot be found on the Allgrand website itself. For the Allgrand Gel 6-CNFJ-150 (12V 150Ah) battery, some data, e.g. a constant current discharge of 15A for all given cut-off cell voltages between 1.6V and 1.8V after 10h, can’t be correct; from the constant power discharge data, the battery might actually have a capacity of 135Ah instead of 150Ah, which means that the adjustment in Fig. 1 would not be correct and the battery would perform better than illustrated. More confusing, Allgrand states “up to 2000 cycles” and “up to 1000 cycles at 30% DOD” in writing which is far lower than the data published as a life cycle graph by the company. While their prices in South Africa are the best of all AGM/GEL batteries, one would like to have a better idea about their performance data.

Surprisingly, the expensive Victron range does not feature with noteworthy cycle life. In general, the spec sheets do not give a lot of information. A number of other batteries show a comparable performance on cycle life.

With the limited information presented here, the best choices for the mentioned requirements are the Omnipower and Oliter batteries. While the Oliter 12V 200Ah is a “genuine” 200Ah battery, the capacities of the Omnipower OPR 120-12 and 240-12 batteries are not 120Ah and 240Ah but 100Ah and 200Ah for a discharge to 10.8V over 10h, respectively. The Oliter with ca. R5,000 is less expensive than the Omnipower which costs ca. R7,000.

I must stress, that I have no experience in the use of any particular battery and merely oriented myself on the available data.

Any views on the validity of the analysis? Any real-life experience to share? I would welcome your comments.

 

 

 


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