Battery Management

[jviss]

Greetings,

I have a couple of 110 Ah wet batteries as my house bank and a group 24 starting battery, wired using the 1-2-BOTH-OFF switch per Maine Sail's scheme.

I'd like to add house capacity, and I have a good pair of 80 Ah AGM batteries.

How can I do this? I was thinking of splitting loads and running house loads like fridge, lights, stereo, etc., off one bank, and instruments, autopilot, radar, etc., off the other.

For charging, the recommended voltages are pretty darn close, so I assumed I'd add an ACR.

It would also be nice to be able to switch banks to loads in case there's a bank fault, similarly to the 1-2-BOTH-OFF scheme above.

Input most welcome.

Thanks,

jv

 

[dlochner]

I think there are some significant issues to consider.

FLA batteries want to be charged at much higher voltages than AGMs, FLAs around 14.8-15.v and AGMs at around 14.4 v.

AGMs can accept a much higher charge current than FLAs and do not tolerate PSOC as well as FLAs do.

How do you plan to charge them?

 

[jviss]

Thanks for your reply.

AGMs...do not tolerate PSOC as well as FLAs do.

Is that backwards?

Anyway, I was just planning on charging them via the combining relay, ACR.

 

[dlochner]

Thanks for your reply.

Is that backwards?

Anyway, I was just planning on charging them via the combining relay, ACR.

The only LA battery that tolerates a PSOC is the Firefly Carbon Foam battery, for the rest, it is a matter of degree. AGMs left partially charged for long periods suffer more than FLA batteries. Neither does well when left below 50% SOC.

The advantage of AGMs is that they accept higher charge rates and recharge more quickly than FLA batteries. So with a high output regulator the batteries can be brought back up to 100% more quickly than FLA batteries.

A better choice would be a battery to battery charger, Victron makes a nice one. Check MS's site, MarineHowTo.com for more details. My information is the readers' digest version.

 

[jssailem]

I agree with Dave. A mixing of the batteries under a single charge controller will compromise one of the sets.

AGM's have the advantage of greater Depth of Discharge - DoD (80%) and quicker recovery. THey allow you to put the battery just about anywhere in the boat. They have a greater acquisition cost. They can be destroyed by poor charging systems which negates all of their advantages.

Flood/Acid are simpler. They are a bit more forgiving. The DoD is the 50% stated by Dave to get optimum performance (which can match upto 98% of AGM performance). True Deep cycle units can give you long life (charge cycles i.e. 700). Are less money to acquire. Can be destroyed quickly if your charging system is not designed to get the batteries back to full SoC. Will take time to charge that last 5%.

 

[jviss]

A better choice would be a battery to battery charger, Victron makes a nice one.

Excellent, thanks!

 

[Benny17441]

How about for recharging underway with the alternator? Is there a plan or a generator aboard?

 

[Stu Jackson]

wired using the 1-2-BOTH-OFF switch per Maine Sail's scheme.

Which one, he has many.

I was thinking of splitting loads and running house loads like fridge, lights, stereo, etc., off one bank, and instruments, autopilot, radar, etc., off the other.

Never a good idea. The largest house bank you can have is the best.

Yes, charging voltages of AGMs and wet cells are close, but not identical, and you already know mixing types is not recommended.

 

[jviss]

Which one, he has many.

Never a good idea. The largest house bank you can have is the best.

Yes, charging voltages of AGMs and wet cells are close, but not identical, and you already know mixing types is not recommended.

The scheme where the switch is in "1" for normal operation, but you can switch to "2" to run the house from the engine batt, "B" to start using the house batt, etc. I wasn't aware of any other schemes.

You can't combine a pair of AGMs and a pair of FLAs into one bank; hence, splitting loads.

 

[jssailem]

You can't combine a pair of AGMs and a pair of FLAs into one bank; hence, splitting loads.

It is not about the loads, it is about the charging of the batteries.
You will need different charging profiles if you want to get the best performance out of both banks. Or you could choose to charge to the lesser bank (I think that would be the FLA's) and know that you are allowing the AGM's to die an earlier death than they would if you charged them as their charge profile requires.

 

[Stu Jackson]

"!" for normal operation, but you can switch to "2" to run the house from the engine batt, "B" to start using the house batt, etc.

I was with you until the "..."B" to start using the house batt, etc...." Do you mean one 1-2-B switch?

I wasn't aware of any other schemes.

Of course he does. Some include 3 I/O switches. Like these, which have been posted hundreds of times:
Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
1/BOTH/2/OFF Switches Thoughts & Musings

This is a newer primer for boat system wiring design with a thorough digram: Building a Good Foundation (October 2016)
Building a DC Electrical Foundation

You can't combine a pair of AGMs and a pair of FLAs into one bank; hence, splitting loads.

John answered that quite well.

Don't know where or why you got AGMs, maybe the price was right, but even separating them into different banks doesn't address the charging issue, (and I already told you why smaller banks aren't good). Either the flas or the agms may suffer, although I do agree that their charging regimens are pretty close.

Maybe another way to approach this, instead of trying to reinvent the wheel (which occurs on this forum quite regularly), is to ask yourself: How come I don't hear about people doing this all the time?

 

[jviss]

I assume when you, @jssailem say "charging profiles," and you, @Stu Jackson say "charging regimens," you are referring to the absorption and float voltages, and maybe the charge termination current, and maybe the maximum current during bulk, but probably not much else, right? Well, do you happen to know what those values are for these different batteries? They happen to be virtually identical. The AGMs can take more current during bulk, but less is not going to cause them "die an earlier death." The recommended absorption voltages are exactly the same, at 14.8V. The recommended float voltages differ by 0.1V. I can't see how a simple ACR can affect the life span of these two banks. But maybe I'm missing something regarding profiles or regimens. I guess one issue could be that your charger doesn't know how the banks are sharing the current, and could terminate the absorption stage prematurely for one bank.

Don't know where or why you got AGMs

Stu, I have them. Why does that matter? For what it's worth, they were from my previous boat, when I was running solar on it. I sold the boat with new FLAs.

John answered that quite well.

While I appreciate John's reply, it's really not correct for the reason I pointed out in this reply. The charging profiles are, remarkably, the same, at least WRT to voltages.

I guess what I'm really trying to figure out is how to take advantage of the extra capacity in two banks of different batteries that I don't want to combine when I'm not charging them! That's why I threw out the idea of splitting the loads on the boat, and I chose the logical split of house versus 'operating,' i.e., navigating, sailing, etc. And then, to be able to select which bank for which, in case there's a fault in one bank or the other. I guess another one or two 1-2-B-OFF switches might be in order?

I mean, even if I bought two more of the ones I have on board already, I still couldn't just add them to that bank, 'cause of their differing ages.

 

[dlochner]

@jviss perhaps you have heard of the KISS principle, you know, the Keep It Simple Silly.

Mixing battery chemistry and charging is the antithesis of KISS and your information is wrong. FLA and AGM batteries require very different charging voltages. You might want to read this article:

How Fast Can an AGM Battery be Charged? - Marine How To

TEST EQUIPMENT USED IN THIS ARTICLE BUY MHT RECOMMENDED TEST EQUIPMENT - AMAZON Baseline Discharge 49.3% SOC PREFACE: This article looks at the *charging times & achieved state of charge of an AGM battery

marinehowto.com

On the other hand, it is your money and your boat. If you want to spend your money on a poorly thought out system, by all means go ahead and come back after a season and tell us how it went. I and many others would like to know how it works out, the cost, and the health of the batteries after a season. If you can provide independent testing data on the batteries before and after installation it would be helpful in supporting your position.

 

[Davidasailor26]

How old are the AGM’s? I know it may feel silly to throw out batteries that still have some life in them, but is it really worth adding the complexity, buying new parts, installing new parts, and running new wire just to use them? 80 ah per battery isn’t even all that much, probably group 24? Why not just recycle them for the core credit and get a couple new Group 31 FLA’s? Maybe the cost will be a little more than the parts you need to get the combined setup working, but it’ll take a lot less time and effort, and result in a system with a lot less points of failure.

If you do still want to go with the AGM’s I would definitely do a 20 hour load test first to make sure they’re not worn out. If they’re depleted and only giving 60aH each then you’re going through all this effort just to add 9% more capacity than a single group 31. Maybe check the existing FLA’s while you’re at it to make sure they’re not worth replacing with fresh AGM’s. If all the capacities are good then you could get the B2B charger and all the switches and wires you need to run the loads the way you want.

 

[JK_Boston_Catalina310]

Maine Sail just added a new article on 1/2/B switches to his Marine How To page. It is his typical great explanation with 3 different wiring diagrams to improve the factory setup on most boats.

Now I will agree with other advice you have been given, using an ACR to charge banks of different chemistries isn't. Great idea. If you are determined to have two different house banks of different chemistry you could us a DC to DC charger.

The first thing you would have to do is split the house loads. This would probably involve adding another panel and rewiring loads the new panel. Then you could install 2 1/2/B switches or 4 on/off switches so that you can power either panel from either bank.

Honestly by the time you do all the work and buy the wire, terminals, switches and second panel you will have exceeded any value left in your current battery bank. It's called the sunk cost fallacy in economics. Just plan a new system scrapping the current and you will be much happier in the end. Read Marine How To's article on the 1/2/B switch and what is a deep cycle battery. If you have the space some of the larger Trojan 6 Volt batteries are awesome. If not, as many T-105s as you can fit.

Good luck

Jesse

 

[jviss]

your information is wrong. FLA and AGM batteries require very different charging voltages.

That's not according to the specifications I have.

I researched "generic" AGM absorption voltage levels and it comes up right around 14.8V @25ºC. For this reply I went back to the exact battery I have, the Battamax BAT-NSAP12-100. The spec says absorption at 14.1 to 14.7. So, at the high end of the spec, 0.1V less than the generic.

The FLA I have is the Trojan SCS-200. The recommended absorption voltage for these is 14.8V.

So, AGM 14.7, FLA 14.8; not "very different charging voltages," but virtually identical charging voltages.

Here are the specs if you are interested:

BattaMax Absorbent Glass Mat (AGM) – Sealed Lead Acid (SLA) Batteries BAT-NSAP12-040 BAT-NSAP12-100

• see page 8.

SCS200_Trojan_Data_Sheets.pdf

• note that Trojan calls absorption "bulk."

 

[jviss]

If you want to spend your money on a poorly thought out system, by all means go ahead and come back after a season and tell us how it went. I and many others would like to know how it works out, the cost, and the health of the batteries after a season.

Nothing I do on this boat is poorly thought out. What do you think I'm doing now, asking for input on this forum? And do you think this is the only source of information I'm using?

That said, I'm not committed to this, but I'm intrigued at the possibilities. I have been interested in splitting house loads from 'sailing' loads for quite some time. I wish I had better terminology for this. By "house" I mean fridge, lighting, stereo, laptop, phone, and tablet charging, water pressure, and I might even throw in bilge pumps and sump pump. By "sailing" I mean sailing instruments: wind speed/direction, depth, speed, multifunction displays, NMEA2000 network and related adapters and converters, autopilot, navigation lights, that kind of thing. I hate when I'm sailing and because all of this is going with the engine off I get a low voltage alarm and have to start the engine. This could, of course, be fixed by increasing the house battery capacity; but at this point, because to make one, bigger bank I'd have to buy all new batteries, so they are matched. I could also buy two more Trojans, but that's $500, and I'd have to manage them as two banks, switching back and forth. So, if I split the loads and figure out how to combine when charging, I won't have to do anything other than charge periodically (and less often when I add solar).

There may be issues with charge combining, I think. I am looking into the Victron DC-DC Smart chargers that I think Don suggested, and also looking into using the two identical Sunpower solar charge controllers I have, one for each bank.

Yes, Don, I agree, it's certainly not simple, and violates the KISS principle. But it's interesting nonetheless.

 

[dlochner]

That's not according to the specifications I have.

I researched "generic" AGM absorption voltage levels and it comes up right around 14.8V @25ºC. For this reply I went back to the exact battery I have, the Battamax BAT-NSAP12-100. The spec says absorption at 14.1 to 14.7. So, at the high end of the spec, 0.1V less than the generic.

The FLA I have is the Trojan SCS-200. The recommended absorption voltage for these is 14.8V.

So, AGM 14.7, FLA 14.8; not "very different charging voltages," but virtually identical charging voltages.

Here are the specs if you are interested:

BattaMax Absorbent Glass Mat (AGM) – Sealed Lead Acid (SLA) Batteries BAT-NSAP12-040 BAT-NSAP12-100

• see page 8.

SCS200_Trojan_Data_Sheets.pdf

• note that Trojan calls absorption "bulk."

Properly named, there are 3 stages to charging, constant current, constant voltage, and float.

The constant current voltage for the AGM batteries you have is 14.4v. Typical constant current voltages for FLA batteries is 14.7-14.8v.

Applying constant current at 14.7 v to your AGM batteries will damage them. End of story.

As I said before, if you're convinced you're correct go ahead. Do a 20 hour capacity test before the installation and one next fall, let's let the data talk.

From Page 8 of the manual.

 

[jviss]

The AGMs are from 2016, were only used one season, and have been maintained. They test nearly new with my ARGUS AA500P Digital Battery Analyzer.

The Trojans were purchase din August of 2018, also test nearly new this year. (I take them home, charge them equalize them in the winter.)

 

[dlochner]

The AGMs are from 2016, were only used one season, and have been maintained. They test nearly new with my ARGUS AA500P Digital Battery Analyzer.

The Trojans were purchase din August of 2018, also test nearly new this year. (I take them home, charge them equalize them in the winter.)

That analyzer appears to be a capacitive tester. These are not particularly accurate. The proper way to measure battery health and actual capacity is a 20 hour test, when a known load is applied to the battery for 20 hours. Here's an example of how it done.

Under-Load Battery Voltage vs. SoC - Marine How To

After 20 Hour Capacity Test = 95.69 Ah Capacity For this test I wanted to: Show the relationship of loaded voltage to SoC on a popular AGM lead acid battery. Utilize a battery not in perfect health to represent real

marinehowto.com