mixing batteries?

[SVJourney]

Hey All,

I am in Papeete with 2 dead batteries and 2each 4 month old batteries that I got in Panama.  ( I should have replaced them all, but thought the 2 that have now died were good)  So here I am in the land of very expensive batteries and they only have AGM.

The question is, what will happen if I mix 2 AGMs with my 2 flooded lead acid batteries?  I know you are not supposed to do that, but I don't know why, or what will happen if I do.

None of my charging systems have switches for flooded VS AGM, they're just a generic alternator and charger that operates off of 110V AC.

Would really appreciate the help, we're just trying to patch the boat together enough to get to New Zealand in 5 months.

Wayne

[slokat]

Other people may have different opinions, but I posed a similar question to my solar subcontractor who sets up systems for cabins and ranches.

His reply was twofold:

Set up charger for newest batteries is first priority.

Set up charger for AGM and use ACR (or other pass through switch) to the flooded batteries, so that they charge after AGM.

While noting that charger won't treat the flooded with an aggressive cycle as they should have, but conversely you will protect the AGM batteries.

It's a common problem in house banks for people that live off the grid and can't replace all batteries at once, or are cycling in new technology.

His other advice was to charge different banks at different times, in the manner that that need to be charged.

So since you can't change settings, charging banks independently might be the simple solution.

[Della and Dave]

Wayne:  I just got done installing a new battery charger mostly because of the issue you are struggling with.  Our old flooded lead acids were no good, and our old charger was probably original with the boat and only had one charging voltage because the only battery available back then was a lead acid.  Our new batteries are AGM, and they have a different charging profile.  Dale and Pete helped me out on a previous post understanding how all this worked.  See the Battery replacement and rewire thread lower down on the Pearson General non-mechanical system etc.....

The new smart chargers vary current and voltage for AGM's during the , bulk charging phase, absorption, then float phases.  This is to match the charging characteristics of the battery.  The voltage, during especially the absorption phase is higher than you would want for a flooded lead acid, and might boil the electrolyte in the battery. 

Our new charger has settings for flooded lead acid and AGM, so it varies the voltage accordingly and has a temp probe on the battery so if it gets too hot, it throttles back.  Unfortunately, it is either AGM for all three banks available, or flooded for all 3, no mix and match. 

If you use the flooded lead acid setting, your AGM's never gets to full charge, so life would be reduced and you would never actually get the full energy out of them you expected, and you might end up accidentally deep cycling them and destroying them because you repeatedly knocked them down too far.  If you use the AGM setting, you overcharge the flooded lead acid. 

In your case, if you use the old style dumb charger or voltage regulator, it will probably work, but my guess is that your new AGM's will never get to full charge because it is set for flooded lead acid voltage max.    Since you don't want to discharge more than 50 % of full charge, you end up limited in the practical capacity you can take out.  Pete or Dale could probably help here. 

For example, if you have a 100 AH AGM, and charge it to 80%, you have 80 amp hour available total.  You only want to take out 50% of full capacity, but that is from the full toped up charge of 100 AH.  50% of 100 is 50 AH, so you really only have 30 AH available in practice.  (80-50)  Essentially you bought batteries expecting 50 AH of capacity and you only got 30.   

There may be a way to split your bus and rewire so you treat them completely independently, but I would have to think about that to see how it would work. 

[SVJourney]

Thanks for the reply guys!
I'm going to have to bite the bullet and buy all new AGM batteries today as the batteries I have left are not even lasting overnight.  Well over $1000.  Not sure what I will do with the charging system, leave it as it is for the moment as I have so many things that are well and truly broken that I am just trying to stamp out fires.  I will go back and read the charging thread again though, that is a great suggestion.

Wayne

[PeteW]

Wayne,

A fully charged Flooded battery measure 12.7V open circuit while a fully charged AGM battery measures 12.8V. Very little difference.    Any charger, alternator or solar charger will charge  either. Charging by definition implies current (amps) flowing into the + terminal of the battery. The voltage that you hold the batteries at (once they are  fully charged) in the Float mode does matter. But even in float mode most chargers push a small current of at least an amp. This explains why its common to see at least 13.5 volts on the battery bank in float mode.

When you say "mix" I think your are really asking if its OK to connect these two battery types in parallel. My answer is yes and no. If your solar charger can maintain a situation where current is flowing into both the AGM and Flooded batteries during the day. Great. But when the sun goes down and the charger stops supplying and open circuit voltage greater than 12.8 V, I'm going to have to say No to keeping them connected in parallel during this period of the day.

This all goes back to the arguments some of made awhile back for never connecting battery banks permanently in parallel. An A/B/Both switching arrangement with your flooded batteries on A and your AGM batteries on B would allow you select both when charging and A or B when supplying power at night.

Naturally I think you would be best off if you switch between AGM and Flooded for charging as well as when running. If you do not have switches for selecting battery banks I would look into doing that. Strapping and unstrapping battery banks day and night will get old. Explains why I wired my banks the way I did. It provides for any configuration and combination of battery types in emergencies.

FYI I have always used a hydrometer for measuring the state of charge in flooded batteries. Its the most conclusive. When all the balls are floating in every cell you're good regardless of what the voltmeter says.

Glad to hear your making the switch to AGM. Good luck  Pete

[SVJourney]

Pete,
I went and read the battery design post again before buying batteries.  Unfortunately, it would have been extremely hard to rewire the boat to handle 4 group 31 AGM's to non parallel service. The battery bank is under the cabin sole and I wanted to get away quick so I replaced with like sized batteries instead of moving the house bank into the cockpit lockers.  If I were in the states, I would have moved and rewired and with new 2V batteries and all for a fraction of the cost of just buying 4 AGM batteries in Tahiti.

In the mean time the new bank is awesome!  I'm loving the power the AGMs deliver.  days and days of amps without having to charge.  (but long generator runs when I do!)

I also realized when reading the battery thread is that there are absolutely no fuses installed anywhere in my battery or charging system at all!  I've owned the boat for 3 years now and didn't even think about that.  Something else I will be taking care of in New Zealand.

Thanks again for all your comments, it helps to be triggered into perhaps saving my boat with the addition of a $5 fuse.

[PeteW]

Wayne,
4 brand new group 31 AGMs in parallel should perform flawlessly. Be careful charging after running them down. That bank when flat presents an incredibly high load to your alternator and your alternator will get hot.  This is where switching banks out to lighten the charging load can conceivably be something you might wish you could do. AGMs can be deep cycled more than 1000 times and I have read that they can recover from stone dead. A fuse link on the alternator might be a good idea. Actually a breaker that you can reset might be more worthwhile.  Fuse links on the batteries are something a USCG courtesy inspection will always catch. You have 4000 amps of short circuit current at your disposal.

Stay safe and well on your voyage.   Pete