Apologies for the lengthy post, but I’m trying to understand the apparent failure of my solar PV batteries in what seems to me to be a relatively short period of time.
Three and a half years ago I bought an off-grid solar PV system from a reputable supplier consisting of two 325W solar panels, a Victron MPPT 100/20 charger, a Victron Phoenix 24/800 inverter and four Exide Marine & Leisure EP800 Dual AGM batteries. The PV system is installed as per the suppliers instructions with the batteries in a 24v series/parallel configuration, and until this last summer has been working well as far as I can tell. It powers all my household (smart) sensors, controllers, PIR bathroom lighting (no windows) and LED nightlights directly from the batteries via the ‘Load’ terminal on the MPPT charger, with a base load of ~1 amp. During the summer, when there is ample solar power, it also powers several other computers from the inverter during the day. When the battery voltage drops to ~24v in the evening the inverter is automatically shut off and the computers revert to mains power by means of an automatic grid/inverter switch. The sensors etc. remain powered from the batteries. As I understand it at ~24v the batteries have reached about 50% capacity and limiting their discharge below this is recommended to maintain the life of the batteries. This typically happens late evening on a sunny day.
The only other addition I’ve made, in order to make use of an Economy 7 electricity tariff, is to add a 50v 200W power supply in parallel with the solar panels, both panels and power supply are protected by 10W series diodes so the higher of the two voltages prevails if both should generate power. The power supply is only on between the hours of 1am and daybreak or 7am, whichever is the earlier, so is not on when the solar panels are generating power. This has the effect of maintaining the battery power through dull days in the winter when there is little solar PV available. I was careful to make sure the MPPT controller worked properly with the 50v power supply before including it, the diode protection being commonly used in industry for redundant power supplies.
I’m now seeing the discharge curve you can see in the attached graph. This graph is with two series batteries only, one of the battery terminal voltage suddenly drops from ~12.5v to to ~10.5v producing the sudden drop from ~25.5v to 23.5v you can see on the graph. Three of the batteries show this discharge characteristic but will charge as normal, the fourth now doesn’t charge above ~10.8v.
Looking at the technical data available online it seems this may be caused by sulfation of the plates in the batteries, but I’m not really sure if this is correct or why it’s happening. Are the batteries permanently damaged or can I recover them in some way ? How can I avoid this if I replace the batteries ? If I have to replace the batteries after only 3.5 years it eliminates any gain I have made from having the solar PV system, other than having a battery backup in case of power cuts.
Any advice that is forthcoming would be much appreciated before I shell out for new batteries that will only last another three years !!
I would test each battery on its own. A single bad battery will do what you are experiencing. Especially with lead acid.
Take each battery out, fully charge them individually, then load test them individually. This will tell you very quickly if one is not performing and dragging the whole bank down.
A simpler but less conclusive test would be to disconnect the battery cables so each battery is by itself. Then individually test the voltages immediately. Then test them again after 12 hrs. A big drop in voltage on any battery is a pretty good indicator it is bad. If a battery is in really bad shape it will lose voltage pretty quickly. This wont be as thorough of a test as a load or capacity test though.
Cycling these daily over 3.5 yrs seems to be about life expectancy. I spent 4 years sailing around the world on a set and after 4 years they were finished.
If you find a bad battery you can, sometimes recover it with a high voltage charge. This should be done with great care and the knowledge of how to do it though. Some chargers like this one Blue Smart IP22 Charger | Victron Energy actually have a “recondition” setting that would be safe to use. I have brought back many lead acid batteries that otherwise would not hold a charge.
True. Especially if they are consistently deeply discharged and/or not sufficiently recharged. In RV applications I generally see 4-6 years as a best case scenario for AGM batteries that are discharged moderately to heavily even if fully recharged. In the automotive world good AGM starting batteries can last 10 years or more because they are never very deeply discharged and constantly recharged if all is working correctly. Like @MikeD said, lithium is the way to go. It will not care how deeply it is discharged and will perform better in a host of other ways too.
Many thanks for the replies, I need to source a modern battery charger, the Victron Blue Smart IP22 charger looks like it would be a good choice as I can try the ‘recondition’ setting and see what happens. With the Exide EP800 800W 95Ah battery which charger would be a good choice, the Victron charger is available in 15, 20 and 30A versions ? Or is it just a case of the lower current charger will take longer to fully charge the battery ?
Of the two batteries that are no longer in circuit, one will not charge above 10.8v so I’m assuming that one has a shorted cell and is probably dead. The other appears to charge Ok, with a cheap car battery charger, and has held a charge with a terminal voltage of 13.83v for more than a week now, so I’m hoping that one may be ok.
I looked at Lithium batteries when I first bought the solar PV system. I opted for lead acid batteries as these were somewhat cheaper at the time and, as the whole system resides in my basement, I was wary of the horror stories about the fire hazard posed by Lithium batteries. If they were in a garage or outbuilding I wouldn’t be quite so concerned, any experience with this risk ? I assume if I were to change to Lithium batteries the Victron MPPT 100/20 charger would need to be reset to work with Lithium rather than lead acid batteries ?
Thanks for the suggestion, I’ve had a quick look at Stuart’s Github page and that looks like a perfect fit for my system, ‘putting in the effort’ is no problem as I’m retired and have plenty of time and enthusiasm for this sort of thing. As well as providing backup for power outages, my solar PV system was essentially installed to support my DIY home sensor/controller system which is Arduino/Raspberry Pi based, so Stuart’s BMS would complement that perfectly
Thanks again for all the comments and suggestions. I think the way forward is to purchase the Victron charger, keep the existing batteries running as best I can for now and look seriously at Lithium batteries and Stuart’s BMS for their management.
Just one further question - what is the real life expectancy of Lithium batteries, as opposed to the data sheet figures ? Of course one advantage of a bank of managed individual cells is that they can be replaced singly if they fail
No, never. Even most LFP batteries don’t manage 6000 cycles. With proper care (i.e. never discharging below 50% SOC), AGM batteries can be expected to last for 800 to 1000 cycles.
Some may have advertised “up to 6000 cycles”. Advertising language must always be translated correctly, and in this case that means: “the cycle count of this battery is definitely below 6000”. And they are correct, even when their batteries average at 300 cycles only.
After a lot of thought I’ve decided the diyBMS is an excellent idea, but a long term project. It fits nicely with my ‘smart home’ system as I already have an EmonCMS system from OpenEnergyMonitor.org.
I’ve purchased a Victron IP22 charger, delivered this morning, I’m planning on trying the ‘recondition’ setting to see what happens. If I can ‘recondition’ the EP800 batteries I’ll keep them charged and available as a back up just in case we get any prolonged power outages.
I don’t know nothing about this brand, but the battery looks like any noname supercheap chinese product.
When buying a battery I would ALWAYS care about the BMS, and I doubt that these cheap batteries have built in anything reliable. Anyhow I would never buy a LFP battery without an active balancer.
When in doubt I would build a DIY battery from EVE cells and a Yixiang DIY box with JK Inverter BMS (see Youtube), but even a DIY battery is not cheaper than the NKON batteries, which are built from very good components (EVE cells, Seplos BMS with active balancer). NKON offers a 48V 16 kWh battery for as little as 1380 €, and 5 kWh is available at 588 €.
Winston cells are eye wateringly expensive, the NKON batteries are more affordable but aren’t available till mid February, and that is quoted as an estimated delivery date. I shall ‘recondition’ the EP800 batteries for now and see what happens while I think about where to go from here.
An update on where I’m at with the solar PV system batteries…
I’ve reconditioned all four of the Exide EP800 batteries using the Victron IP22 charger, two show little change, after charging they drop to ~10.8v within an hour or so with a 5W LED lamp load, the third is improved and lasts quite a bit longer with a 1 amp load, but still drops to ~10.8v after several hours, the fourth is showing no problems. The best two batteries are in the system at present and are lasting from sundown at ~5pm till around midnight before showing the sudden drop in voltage. Is it worth, or sensible, to try reconditioning the poor batteries more than once ?
One thing I have noticed, when the ‘good’ batteries have been run down for a while and the solar charger comes up in ‘bulk’ charge mode it applies 28.3v across the two batteries for a short period (usually 30-40 minutes) before dropping back to ‘absorption’ mode on 27.1v. During the ‘bulk’ charge mode the good battery has up to 16v or more across it for a short period before the voltages across the batteries stabilise at about the same level. Once charged the two batteries are within 3-400mV of each other. I assume applying 16v across the good battery is effectively ‘reconditioning’ it as that seems to be what the Victron charger does in recondition mode ?
I’ve dismissed using Lithium batteries for now, I can’t justify the expense at present for decent quality, non Asian, batteries. I’ve opted to purchase four ‘wet’ leisure batteries, locate them in an outside cabinet and re-locate the solar gear onto the inside of the wall behind the cabinet. I can purchase ‘wet’ batteries with a four year guarantee quite cheaply and replace them several times for what I’d pay for Lithium at present. Hopefully in four years the price of Lithium may have dropped considerably and I’ll have had a chance to get to grips with the diyBMS system.
Thanks to all who have replied to my original post, I’ve learnt quite a bit from you all.
Sorry, but a charging efficiency of only 10% for lead-acid batteries is far from reality. The charging efficiency is typically around 80-90%.
That’s a thing of the past. Here in Germany, we as end customers can buy Chinese LFP cells for around €80/kWh. That’s far less than we would have to pay for lead-acid batteries, especially since lead-acid batteries only have half the usable capacity.
One can also look for used car batteries like these VW/ID 8s 6.85kWh and with a decent BMS. Some times one can find some cheap ones.
I use them in my project @How sustainabile an ESS can be? and after 2Years i can see no degradation at all. These high quality packs much more safe as an DIY pack off 18650. The two internal temperature elements fit to JK BMS or Dali (others i have not checked).
