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Improving charging and DOD performance with AGM batteries


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Folks - compliments of the season to you all. Firstly this is a great resource to share experiences and to learn at the same time of improving system performances 

in late 2018 I Investec in a basic ‘Axpert’ 3KVA 24VDC inverter and ran the system initially with two deep cycle lead acid batteries (Enertec). I replaced these in mid 2022 with two Vision RCT VLRA AGM batteries. Admittedly there are better solutions out there but this is what my budget allowed for. I’ve done the integration myself along with providing remote status indicators via the G-ultra platform. see pictures attached. The current firmware is U1 02 71. 

The system does what it is expected to do though if I look at the data sheet for the batteries - the standby charging voltage indicated is 2.22V per cell which is different to the requirement stated at 2.4V per cell for cyclical usage. The AGM setting on the Axpert aligns with the standby operating requirements for the batteries.

I endeavour to keep the batteries around 12v each after a two hour LoadShedding instance. It is possible to get four hours out from the batteries though this is really is only possible with doing load management which I’m achieving with a bunch of smart SONOFF’ switches. The load management I believe also helps to keep the DOD managed during two hour outage though ons also has to ask family members to respect that the system has inherent limitations. 

When Utility power returns - I am concerned that the inverter fails in some instances to fully recharge the batteries. In many  cases I’ve seen the inverter charging the batteries over an extended period of time and the batteries do become noticeably warm which I take is normal behaviour. In other cases- the voltage is kept at 27V for about 10min after utility supply return and the inverter obviously assumes that they are charged. I’ve heard the internal fans start up - without there being any Charing activity on the inverter and/or batteries and these run for 5min a time. 

A dumb question- is it possible to optimize the charging of these batteries so as to extend their useful lives? 




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After some research on the topic - I’ve considered the following information and acted accordingly;

1) it is a known problem with the 1 and 3kVA Axpert inverter and unfortunately cannot be remedied (there is no patch for the firmware available). 
2) I have changed the settings of the charger from AGM to USE which already has improved the per cell voltages on the batteries. 
3) I have rotated the two batteries in the system as I noted that the voltages across the terminals were not uniform. 

If there is any other information I I should be aware of I would be happy to receive it 



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29 minutes ago, VaughanT said:

Budget is a little tight at this point. The Narada 200AH 12V AGM batteries work quite well ~ but do come at a price point 

Just my take on getting the AGM to last longer. Try not to charge at over 15% of the Ah capacity. Unfortunately this will extend the charge time and you will end with less capacity for LS. 

Fans also come on when using more than a set value from battery power. In my case over about 130W.

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To be honest, there really isn't much more you can do that you already are... While it can be a pain, caring for batteries is a time and effort intensive nuisance.

I personally tend towards "USE' or user defined battery parameters so you can define all the voltages and you can set the bulk right, I have found that the default bulk voltage is usually a little low so the batteries don't get a full charge reducing the number of cycles.

Otherwise, you can look into a BMS type situation... not only will it make it easier to baby your batteries (Most come with an app or something like that) but it will give you better feedback on your battery behavior via graphs etc. 

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Good afternoon Vaughan,

Compliments of the season to you too.

I have a similar setup to you in terms of size and age. 

My Must 3kva, 24v inverter is hooked up to 4 x 205ah SLA Trojan batteries. These were my second set of batteries, my first set (2 Chinese 280ah) although first use when installed had stood for a few years without any care so didn't last long coupled with my poor understanding at the time of how to use and care for LAs.

I went the Trojan route as at the time I couldn't find a decent 24v lithium option and decided to get decent lead acids as a compromise.

I installed them with an HA02 balancer and I also purchased a Victron BMV712 which I used as a warning a cut off to ensure the batteries don't discharge more than 40%.

I have also taken to adjusting the bulk charge and float voltages twice a year to deal with the temperature fluctuations based on the data sheet for my particular batteries.

It was interesting that when I installed the BMV I found a 0.7v difference between what the inverter was reading the battery voltage at and what the BMV saw, the BMV was accurate the inverter was not, so I used that info to adjust the charge parameters accordingly so I got voltages at the terminals in line with the battery data sheet.

This set of batteries are coming up on 3 and a half years old now with no significant loss in performance, although they are Trojans and should mange that easily. The next 3 years will tell me if what I have been doing is of use or just a waste of time!

Lastly it does take a long time for the batteries to get fully charged, from say 70% SOC it takes probably 5 hours to get to where they are not accepting any further charge.


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This is excellent feedback- the fact that you’ve invested the time and energy to and got stretch the lifespan of the batteries is commendable. 
My first batch of batteries was the Enertec deep cycle which I pushed to get three years out of. The float bug contributed to their demise. They would not have survived the present LoadShedding schedules in any event 

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