General Battery Charging Question - Partial State of Charge

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outrigger999
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General Battery Charging Question - Partial State of Charge

Post by outrigger999 » Sat Jun 27, 2015 11:52 am

HI Folks, I was wondering if someone could educate me on what I should do in the following situation...

The normal preferred off grid battery system cycle is the following:
1. Batteries get charged fully charged during the day from the solar array + household daytime power needs are met as well.
2. Sun goes down and household is fully on batteries
3. we make it through the night until the sun comes up again and you repeat step 1.

In the case where overnight I don't make it through the night (I hit 50% DoD) I switch automatically over to my local power company which sits in the generator position. At this time, ONLY the household power needs are met until the sun comes out again and then I get back on the cycle I mentioned at the top where I charge both the batteries and provide for the household via the solar array.

Here's the question: When the batteries are in a 50% charge state, is it ok for them to sit there for potentially several hours until morning? If you're using HBX, the manual suggests that's what you should do. In my case, I'm using Gen Alert to achieve the very same thing as HBX and when I switch over to the power company, I DON'T charge my batteries off of the power company but let them sit until morning when the sun comes out. Economically, this is the best solution of course since I live in Hawaii where 1kW/h = $0.3544 which is the highest in the nation (some of the Hawaiian outer islands are actually higher but I'm on Oahu). In any case, I have the charger on my VFX3648 inverter turned to 0 amps so that it doesn't charge my batteries off the power company. I've heard that leaving batteries in any partially charged state for any amount of time is bad for them so I ask the community at large what you think?

I could instead of performing a full charge when on the power company, allocate some amps (perhaps a low amount) so that the batteries trickle charge until morning if that's recommended but I'd prefer someone telling me what should be done and why.

Just for completeness, the batteries in question are Outback Power RE200 AGM batteries.

Thanks for your help in advance.

Vic
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Re: General Battery Charging Question - Partial State of Cha

Post by Vic » Sat Jun 27, 2015 4:37 pm

Hi outrigger,

It is not clear just how much PV power that you have, but, IMO, it seems that you do not have enough battery Capacity, if you cannot make it through some nights on battery power.

Determining the 50% SOC point is not simple, as if one uses battery voltage, it is affected by loads at the time of LVD, and the loads in the preceding few hours, temperature, and so on.

AGM batteries prefer to not be too deeply-cycled verses Flooded batteries, for example.

What is your PV array's STC power rating, and are your AGM batteries in a single string?

Some opinions, FWIW. Thanks for any additional info, Vic
4/20/08: 18 Shell SQ 175-106 Vmpp, Stacked 5548 SW+, 1350 AH Surrette 4KS25's, MX-60, Kubota SQ-3250 25 KVA Polyphase Diesel genset. Thanks OutBack for this Forum + the great Support and Service.

outrigger999
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Re: General Battery Charging Question - Partial State of Cha

Post by outrigger999 » Sat Jun 27, 2015 6:03 pm

Hi Vic,

You are correct, I don't have enough battery power for what I need to make it throughout the night all the time; although the reason for that is beyond what I'd like to explain here. I knew that was a possibility going in since I will add to the system this year and it was always a 2 phase installment for me since as you know PV + batteries are expensive. Just to give a little bit more detail, I have 8.2kW of solar, and 712AH of total battery capacity at the C20 rate. So given that I know I don't have quite enough capacity and that I can make it through the night but not always, I'd like to make sure that my original question is answered...

What is better for the batteries when they reach 50% DoD? Can I let them sit until morning if there's a couple of hours between hitting that or should I immediately apply at least a trickle charge? Perhaps people think I should charge them fully or perhaps apply more than just a trickle charge? If you have an answer, please tell me why as well so I can be educated. My inclination is to leave the batteries in that state until the sun comes up again since this is also an economic concern however if it means long term damage to my batteries I will do what I need to so no harm comes to them. Keep in mind that at the cost/kWh here in Hawaii, it's not unusual for people to have $400-700 electric bills every month so the cost is very real in terms of impact.

Since you brought up DoD and the many variables around that, let me just say again that you're right but it's human nature to want to distill that down to a reasonable, fairly straightforward answer. For the Outback Power RE 200, the answer from Outback's tech support folks from several conversations on this topic is the following:

50% DoD for a 48v system is 46.3v under load and 48v not under load. Once you reach low voltage on your system and remove the load from your batteries, they should rise back up to 48v in around 15 minutes or less. Please let me know if you've heard differently or if you have another way to calculate DoD; again with an explanation for why you do it that way. I do have an RTS in my system so temperature is taken into account. As you can imagine, in Hawaii, the temperature here is on the high side so I do everything I can to insure the batteries are as cool as possible and well ventilated.

Thanks, Scott

Vic
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Re: General Battery Charging Question - Partial State of Cha

Post by Vic » Sat Jun 27, 2015 7:14 pm

Hi Scott,

Yes, I did not exactly answer one of your questions.

I have not ever run AGM batteries, as they are not appropriate for my off-grid system goals. But, personally, I would not discharge any systems that I look after as deeply as you appear to be doing, especially not with AGM batteries. Deeper discharges require long absorption times, and this is probably what limits the lifespan of AGMs (long absorption stages consume the catalyst recombination elements).

Similarly, I would not routinely discharge even the FLA batteries here to 50% SOC. Do this deepish discharge, purposely, on occasion, but then IMMEDIATELY begin recharge within minutes of reaching a nominal 50% SOC.

So, the only answer to your question about the immediacy of recharge when reaching 50% DOD, I cannot answer, as, I have no direct experience with AGM batteries, which differ from FLA batteries in several important ways. And, there are a number of ways to make AGM batteries.

Probably the major reason for not letting LA batteries sit at a relatively low SOC, is due to concerns about allowing sulphates on the plates to harden, making it more difficult to remove by simple recharging. Have read, that regarding FLA batteries, that 50% SOC is about the limit that one would want to discharge common Flooded batteries, and, at that, lot let them sit at that SOC. Forklift batteries appear to be able to tolerate discharges to 20% SOC, nominal, but then they definitely need to have recharge begin immediately, and at a fairly high charge Rate.

Would expect that OB is the place to ask your detailed question about the need to recharge, with what time delay, and at what Rate the recharge should occur, and so on.

One issue with a relatively low SOC in the morning, is the possible chance that one can get behind in recharging, particularly, if following days are a bit cloudy, or if loads increase, and so on. Personally, even 60% SOC in the morning would be a bit concerning, but there are many ways to design and use PV charged battery systems.

Still, to me, the voltage of rebound of a battery when a load is removed, still depends on the magnitude of the load, and the length of time that the battery (that is not being charged) supplied the load (as well as the battery temperature). Agree that one can get a rough idea of the SOC of a battery based upon voltage, but this approximate SOC is nothing that I would want to hang my hat on.

All FWIW. Normally, detailed application questions about batteries really should be asked of the manufacturer, or when necessary, the supplier/rebrander. Just my opinions. FWIW. Good Luck, Vic
4/20/08: 18 Shell SQ 175-106 Vmpp, Stacked 5548 SW+, 1350 AH Surrette 4KS25's, MX-60, Kubota SQ-3250 25 KVA Polyphase Diesel genset. Thanks OutBack for this Forum + the great Support and Service.

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Re: General Battery Charging Question - Partial State of Cha

Post by SandyP » Sat Jun 27, 2015 7:26 pm

Vic wrote:detailed application questions about batteries really should be asked of the manufacturer, or when necessary, the supplier/rebrander.
Yes, this is probably the best advice / answer.

You could also see if there is any relevant information on a boat-sailing forum where they talk about their "house batteries".
For "off gridders" in general, battery information in sailing / boat forums is probably the most relevant as they are also Off Grid and generally in a worse situation as they probably have much less recharge ability when away from the dock.

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Re: General Battery Charging Question - Partial State of Cha

Post by larrywa » Sat Jun 27, 2015 8:35 pm

outrigger999 wrote:HI Folks, I was wondering if someone could educate me on what I should do in the following situation...

....
In the case where overnight I don't make it through the night (I hit 50% DoD) I switch automatically over to my local power company which sits in the generator position. At this time, ONLY the household power needs are met until the sun comes out again and then I get back on the cycle I mentioned at the top where I charge both the batteries and provide for the household via the solar array.

Here's the question: When the batteries are in a 50% charge state, is it ok for them to sit there for potentially several hours until morning? If you're using HBX, the manual suggests that's what you should do. In my case, I'm using Gen Alert to achieve the very same thing as HBX and when I switch over to the power company, I DON'T charge my batteries off of the power company but let them sit until morning when the sun comes out. Economically, this is the best solution of course since I live in Hawaii where 1kW/h = $0.3544 which is the highest in the nation (some of the Hawaiian outer islands are actually higher but I'm on Oahu). In any case, I have the charger on my VFX3648 inverter turned to 0 amps so that it doesn't charge my batteries off the power company. I've heard that leaving batteries in any partially charged state for any amount of time is bad for them so I ask the community at large what you think?

I could instead of performing a full charge when on the power company, allocate some amps (perhaps a low amount) so that the batteries trickle charge until morning if that's recommended but I'd prefer someone telling me what should be done and why.

Just for completeness, the batteries in question are Outback Power RE200 AGM batteries.

Thanks for your help in advance.
Your plan, considering the hardware you have, sound about the best to me. IIRC they recommend less than 40% DoD for the usual usage with 50% OK on occasion.
The sulphation of the plates is dependent on the accumulation of time spent at a lower charge and how much lower, so it is not something that happens suddenly but rather gradually over time. Best o avoid and your back-up plans using the grid in minor amounts, at off-peak hours too, to save your batteries sounds good.

Where I am batteries would be a disaster as we can go December and January without any energy production some years. If the sun does come out one day the panels are half a metre deep in snow anyway. I already ruin two sets of batteries off grid while I was building my home although a runaway windturbine baked one set. Eventually the blades flew off and I still haven't found the pieces of one. Several pieces of one were found in my attic through a 1m hole in the roof. :) Yeah we get 120-130kph winds on this hill, frequently. Enough of that nonsense.

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Re: General Battery Charging Question - Partial State of Cha

Post by csteel » Mon Jun 29, 2015 6:39 am

Sometimes the simple answers are often overlooked. Look at ways to reduce the nighttime loads first, perhaps using timers. I am in a high desert, very hot in the daytime cool at night, normally a 40 + temperature deviation. One method commonly used here is to shut down the freezer with a timer at night this is usually not a problem as it does not get opened when you are sleeping. As I recall my wonderful time in Oahu there is almost always a breeze, maybe a small wind unite to trickle the bank at night. Israel "IZ" Kamakawiwo╩╗ole is still alive in my computer. Chuck

outrigger999
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Re: General Battery Charging Question - Partial State of Cha

Post by outrigger999 » Tue Jun 30, 2015 1:01 pm

Today I called Outback and asked them two questions:

1. What is the 50% DoD mark for the RE200 batteries and...
2. Did I need to apply a charge immediately after the Gen Alert or HBX switch over from batteries to AC (or generator)?

They told me that 50% DoD for these batteries is 47.2v resting. That is under load, you can bring the batteries down to somewhere around 46-46.3v (depending on the amount of load) and when you pull the load off, the voltage should raise back up to 47.2v or above. If you don't find it 47.2v or above you need to adjust the low voltage trigger and the time delay.

They told me that I did not need to apply a charge (trickle or otherwise) to the batteries in the interval between hitting the 50% DoD and the sun coming back up for another cycle. The interval here is only a couple of hours so there's no harm and these batteries have a very slow self discharge rate.

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