What problems can be created by batteries with a MINIMUM charge current recommendation

mevenable · January 2020

The batteries that I originally bought have developed problems in less than 2 years and are no longer made, and the company is offering to replace them with different batteries, North Star Blue. However, the original Outback 106NC batteries had a maximum charge current of 30 A, and these have a minimum of 36 A. My system at best produces 36 A in the middle of the day, and I use it for some irrigation and so there is always a load of about 15 to 20 A. I charge the current batteries with 5-15 A and they do fine. What problems might be incurred by bringing in a set of batteries that recommends a minimum charge current to 36 A?

mcgivor · January 2020

mevenable said:

The batteries that I originally bought have developed problems in less than 2 years and are no longer made, and the company is offering to replace them with different batteries, North Star Blue. However, the original Outback 106NC batteries had a maximum charge current of 30 A, and these have a minimum of 36 A. My system at best produces 36 A in the middle of the day, and I use it for some irrigation and so there is always a load of about 15 to 20 A. I charge the current batteries with 5-15 A and they do fine. What problems might be incurred by bringing in a set of batteries that recommends a minimum charge current to 36 A?

Being that the most common cause of battery failure is undercharging, it's likely that was the cause of short lifespan of the original set. One would need to subtract the load current from the charge current along with all other potential losses, running a deficit will lead to dissapointment, not only in battery lifespan expectancy, but also financially, it would be unlikely they would be replaced twice under a single warranty.

Should the current array not be able to meet the charging requirements of the previous set, it would be a wise decision to increase said array, possibly charge controllers to not only meet, but exceed the requirements to protect against low solar input. Consider yourself lucky that the replacements are offered, not all are fortunate to be given a second chance.

mevenable · January 2020

actually, the system was designed and installed by an outback approved installer, and reviewed on site by an outback trainer, and is fine, optimal even, for the old 106 NC outback batteries. They had a MAXIMUM charge of 30 A, and no minimum. They were not supposed to be charged at high current, and modest current was best. The batteries were usually fully charged by early afternoon, with around 2 hrs of absorption, and floated after that.

The batteries in question are not actually being offered under warranty, which ran out a year ago, but are kindly being offered in a win-win deal since they want to introduce these into Nicaragua, where I live. They used to sell the 106NC here, and now they want to introduce the new North Star blue that is their new line. However, they are providing the batteries in the US, and I have to get them to Nicaragua. Once here, if any of them give problems at least at present I have to bring in replacements from the US (maybe eventually they will get someone to sell them here, that’s the idea). It is actually only one of my 12 batteries that currently has a problem, but that reduces me to two strings, and it is proving very difficult to find a substitute for that one battery. The whole thing functioned very well until that one battery developed a problem a couple months ago.

Thing is, unlike the 106NC that has a maximum charge current (and other batteries I can get still in Nicaragua) this style has a Minimum charge current. I have no intention of adding to my 15 solar panels and FM80 CC, which are ideal for my needs. Btw, one reason I charge my current batteries with 5 to 15 A is that I irrigate during the day and run the pump which takes 15 A or so. This keeps my system from using too MUCH charge for my current batteries.
So, I am trying to figure out if it is worth the trouble and expense of bringing in these batteries, even if they are “free”, and facing issues of replacement from the US if one goes bad, especially if they won’t charge adequately. I asked the outback battery guy who is offering them, but he doesn’t understand the problem, doesn’t seem to see one. I am still clarifying my question with him.

Either way, if I buy here or bring those, I am probably switching from three strings of 100 Ah to two strings of 200 Ah.

Marc Kurth · January 2020

FWIW: Concorde AGM batteries have a specified minimum charge rate of .20C when the batteries are deep cycled. Their maximum charge rate is extremely high. Typical PV systems are incapable of producing .2C but they are also not normally cycled deeply either. Their reasoning is based on the batteries spending too much time in a depleted condition, resulting in sulfation. Undercharging rears its head in many forms.

Take your question to North Star with specific emphasis on your average daily DOD and charging parameters. I am going to forecast that their answer will that you are fine with reaching float early afternoon.

Marc

Estragon · January 2020

These appear to be designed more for backup (eg telecom) service. This type of battery isn't generally well suited for off-grid, but this particular one may be. IDK.

NSB BLUE+ Battery® Application Manual

Looking through the doc (and assuming this is the battery being proposed) one problem may be the fairly low recommended absorb ("boost") voltage of 2.35vpc at 25°C (14.1v for 12v block). A thin plate AGM can accept fairly high bulk charge rates, but a lower voltage absorb will take longer to finish the charge. With solar, a low rate bulk plus a long absorb may not get finished before sundown. A higher absorb voltage would finish faster, but the design likely limits how much higher you can go without risking damage.

They suggest absorb be limited to 5 hours. By implication, this means you should be able to supply enough current to carry loads plus charge to ~80-85% SOC by ~5hrs before sundown.

I don't think charging at below recommended bulk current is the issue per se. Any net current into the battery will eventually fill it. The issue is whether it will do it in the time available.

Dave Angelini · January 2020

Yep, in the time available and on a very reliable basis in poor solar weather for offgrid. Poor conditions should still have a decent output. More solar panels! Being an approved installer means very little (he filled out some forms) and due diligence is always required offgrid.

mevenable · January 2020

Like I say, the 106NC batteries they installed got charged for the most part without problems, plus they were supposed to be for systems that didn’t get fully charged some of the time, although that was maybe 5% in our case. I am just not sure if these North Star batteries are a good fit, and whether they might be more for grid charged back up. Since my present array fits my needs, the expense involved in change essentially everything just to use these “free” batteries doesn’t seem sensible. Either they work with what I have, or they don’t and I get something else here in Nicaragua. That is the decision.
i would try substituting in a different battery for the one that went bad and going on with the present set, but I have been unable to determine what locally available alternative would be a good fit. I have not been able to work out a way to measure the internal resistance of the used batteries I have, and neither outback nor the company that made the batteries (and still makes them under another label) can even tell me what the internal resistance was when new (I could try guessing from that). While I could use my car headlights and taillights to measure voltage under different currents and do the math, wiring them in seems complicated. I have been going round and round with this. I know 5he internal resistance of every battery available in Nicaragua, since most companies put it in their data sheet, but not 5he ones I have.

Estragon · January 2020

As Marc suggested, I'd ask North Star about suitability in your specific application, with details on bank size, DOD, etc. It may be that increasing absorb voltage, for example, to 1.4ish vpc would allow a reasonable absorb time of say 2-3hrs.

What problems can be created by batteries with a MINIMUM charge current recommendation

Comments

Categories