Different amp hour batteries?

[tinman01]

Hi, My challenger has two 6 volt 75 amp hour batteries for the coach and I just added 4 - 6 volt 225 amp hour trojans t105. I have been told that having the two 75 amp hour batteries hooked up with the 225 amp hour ones will degrade the 225 amp hour ones and they won't charge correctly. They are all flooded batteries.

At some point I will replace the 75 amp hour ones but right now don't have another use for them. They not even a year old yet so I hate to just get rid of them but I don't want to hurt my new batteries.

Is there some way of doing this that will make use of the old batteries, and the XM1800 watt inverter as well, I will be replacing it too with a Aims 4000 watt pure sine.

Any advice will be greatly appreciated.
Thanks
Don

[Chance]

Your drawing needs a revision to show 6-Volt batteries in series on the Trojans.


Are you sure about existing 6-Volt being only 75 Amp-hour? That's a very low battery rating for a 6-Volt.

[16ACE27]

Originally Posted by tinman01

Hi, My challenger has two 6 volt 75 amp hour batteries for the coach and I just added 4 - 6 volt 225 amp hour trojans t105. I have been told that having the two 75 amp hour batteries hooked up with the 225 amp hour ones will degrade the 225 amp hour ones and they won't charge correctly. They are all flooded batteries.

At some point I will replace the 75 amp hour ones but right now don't have another use for them. They not even a year old yet so I hate to just get rid of them but I don't want to hurt my new batteries.

Is there some way of doing this that will make use of the old batteries, and the XM1800 watt inverter as well, I will be replacing it too with a Aims 4000 watt pure sine.

Any advice will be greatly appreciated.
Thanks
Don

As long as the chemistry of the batteries is the same (Lead-Acid in this case) I see no way the lower AH batteries can affect the charge ability of the higher AH batteries.

[tinman01]

Originally Posted by Chance

Your drawing needs a revision to show 6-Volt batteries in series on the Trojans.


Are you sure about existing 6-Volt being only 75 Amp-hour? That's a very low battery rating for a 6-Volt.

Yes they are in series (I know I just noticed it myself, it's been a long day! LOL), but the top two trojans are in series (Pos to Neg) and the bottom two are in series (Pos to Neg) and the two Deka are in series (Pos to Neg), then all are in parallel (Pos to Pos and Neg to Neg). I don't actually have them hooked to the Deka's at this time, the Trojans are in the motorhome and hooked together but I didn't hook them to the coach yet, When I had my rig serviced they did something to my auto gen start and it quit working and I didn't want to add anything or change anything until they fixed their trouble first. (didn't want them blaming the new batteries!) so I'm waiting for it to come back from the shop before I hook them up, but they are in the coach and wired in series and parallel together just not to the Deka's yet.

That's when I came across the info about it not being a good thing to mix amp hour batteries, even though they are all flooded cells and thought I better check on it before hand.

What I heard was the charge controller will see the 75 amp hour batteries full and stop short of fully charging the 225's.

[SuperD]

The 225's wil try to equalize with the 75's and that's not good for either set. I know they aren't cheap but you need to ditch the 75's and get two more 225's. Lifespan of the 225's will be shortened considerably.

[tinman01]

Originally Posted by SuperD

The 225's wil try to equalize with the 75's and that's not good for either set. I know they aren't cheap but you need to ditch the 75's and get two more 225's. Lifespan of the 225's will be shortened considerably.

Thanks, I am replacing the 1800 watt modified sine with a 4000 watt pure sine and will be adding most everything in the coach (not AC) to run off the battery bank and I think your right, it will be better to take them out of the coach.

That will leave me with a separate battery bank with inverter to play with. I'm open to some ideas for them, any suggestions?

Thanks for the help
Don

[TyCreek]

Put an isolator between them and your main bank to use as a supply backup. Or box them with the extra inverter to make a portable 12V/AC unit. Or sell the extras ... after you're done playing of course!

[Chance]

Originally Posted by tinman01

.....cut.....

That's when I came across the info about it not being a good thing to mix amp hour batteries, even though they are all flooded cells and thought I better check on it before hand.

What I heard was the charge controller will see the 75 amp hour batteries full and stop short of fully charging the 225's.

That's the generally-accepted rule of thumb that's often quoted, partly because it's so simple that it always works, but it doesn't take cost into account. If all batteries are the same model, size, age, and rating, then life is simple, right?

So what happens if you had 6 identical batteries and after a year or two one of the batteries fails. Now they would be different ages. And what if exact model is discontinued? Do you replace all six? I certainly wouldn't. It can't be that black and white in my opinion.

By the way, I think your 75 Amp-hour 6-Volt battery ratings are likely a mistake (they would be physically tiny -- like roughly 1/3 the size of the Trojans) but we'll go with that.

Your controller can't see that the 75 Amp-hour batteries are full (unless you have a very sophisticated system I'm not aware of) because the controller only sees the battery bank as a unit. In this case it sees 525 Amp-hours at 12 Volts. As a simple example, think about charging the three strings from an alternator that puts out a constant 14 Volts. Excluding cable resistance, each string will see 14 Volts regardless of what charge current each string accepts. Each string would charge at its own rate based on 14-Volts.

I think the biggest problem with mixing batteries of different types comes in because the internal resistance of each battery string that is wired in parallel (in your case 3 strings with two being the same and one different) is not always the same, or more importantly the resistance is not necessarily in proportion to each string's Amp-hour capacity.

Basically the string with less resistance will receive more current (in this case likely your Trojan batteries) while the Deka will probably get less charge current. But if they get 1/3 as much charge current for 1/3 the Amp-hour, what's wrong with that? I think that's what Tfryman was implying. And it could be correct because the battery resistance should be similar per unit size if the chemistry and basic design is similar.

However, real deep cycle batteries are designed and manufactured differently than starting batteries or dual purpose batteries, which means their internal resistance can be different, even when adjusted for size/capacity.


Bottom line is that the Deka batteries "may" charge at same rate (adjusted for capacity) than the Trojans. Or not. You could always test it to see if there is a significant difference. If there is, I wouldn't use them -- particularly if they are only 75 A-h versus Trojan at 450 A-h.

Again, I question the 75 A-hour rating. If that's all they really are, I wouldn't risk it for such minor additional capacity.

With a multi-voltage converter the problem is obviously a little more complex, but what would remain important to me is whether the 3 strings charge at similar rate or not.

[16ACE27]

Originally Posted by Chance

That's the generally-accepted rule of thumb that's often quoted, partly because it's so simple that it always works, but it doesn't take cost into account. If all batteries are the same model, size, age, and rating, then life is simple, right?

So what happens if you had 6 identical batteries and after a year or two one of the batteries fails. Now they would be different ages. And what if exact model is discontinued? Do you replace all six? I certainly wouldn't. It can't be that black and white in my opinion.

By the way, I think your 75 Amp-hour 6-Volt battery ratings are likely a mistake (they would be physically tiny -- like roughly 1/3 the size of the Trojans) but we'll go with that.

Your controller can't see that the 75 Amp-hour batteries are full (unless you have a very sophisticated system I'm not aware of) because the controller only sees the battery bank as a unit. In this case it sees 525 Amp-hours at 12 Volts. As a simple example, think about charging the three strings from an alternator that puts out a constant 14 Volts. Excluding cable resistance, each string will see 14 Volts regardless of what charge current each string accepts. Each string would charge at its own rate based on 14-Volts.

I think the biggest problem with mixing batteries of different types comes in because the internal resistance of each battery string that is wired in parallel (in your case 3 strings with two being the same and one different) is not always the same, or more importantly the resistance is not necessarily in proportion to each string's Amp-hour capacity.

Basically the string with less resistance will receive more current (in this case likely your Trojan batteries) while the Deka will probably get less charge current. But if they get 1/3 as much charge current for 1/3 the Amp-hour, what's wrong with that? I think that's what Tfryman was implying. And it could be correct because the battery resistance should be similar per unit size if the chemistry and basic design is similar.

However, real deep cycle batteries are designed and manufactured differently than starting batteries or dual purpose batteries, which means their internal resistance can be different, even when adjusted for size/capacity.


Bottom line is that the Deka batteries "may" charge at same rate (adjusted for capacity) than the Trojans. Or not. You could always test it to see if there is a significant difference. If there is, I wouldn't use them -- particularly if they are only 75 A-h versus Trojan at 450 A-h.

Again, I question the 75 A-hour rating. If that's all they really are, I wouldn't risk it for such minor additional capacity.

With a multi-voltage converter the problem is obviously a little more complex, but what would remain important to me is whether the 3 strings charge at similar rate or not.

What you are saying is correct except for one thing: "Internal Resistance" really has no bearing on charge rate. Sure, internal resistance does impede both charging and discharging current and creates heat in the battery, but what determines the amps going into a battery is the difference between the source voltage (the charger) and the battery voltage. When the battery voltage increases, charging current will decrease. When charging batteries in parallel, as long as they have the same chemistry they will all have the same "full" battery voltage. Some of the batteries may reach that voltage before others, even if they all are the same make/model/AH/age. The ones that reach that voltage first will have almost no current applied to them, the lower voltage batteries will be receiving the charging current. When the charging circuit senses charging current to all batteries has decreased to its pre-set value, it will kick the charging voltage down to the "maintaining" voltage; normally about 13 volts.

[gmc]

Remember there are 2 sides... Charge and discharge...

On the discharge side, properly wired, you will draw equally from all batteries... And the lower capacity cells will draw down quicker.. The batteries will try to equalize drawing from the higher capacity..

On charge side, not great either as you are likely to overheat the lower capacity... The charge voltage won't drop as quickly as the lower capacity batteries would like. Not as bad with flooded as some other chemistries, but still not good.

Bottom line, to me not worth the risk to the new batteries.

[16ACE27]

Originally Posted by gmc

Remember there are 2 sides... Charge and discharge...

On the discharge side, properly wired, you will draw equally from all batteries... And the lower capacity cells will draw down quicker.. The batteries will try to equalize drawing from the higher capacity..

On charge side, not great either as you are likely to overheat the lower capacity... The charge voltage won't drop as quickly as the lower capacity batteries would like. Not as bad with flooded as some other chemistries, but still not good.

Bottom line, to me not worth the risk to the new batteries.

Nope, lower AH batteries will supply less current than the higher AH batteries. All batteries will deplete equally by capacity %. All batteries are tied together in parallel so they will all be at the same voltage. The lower AH batteries can not get to 12.2 volts while the higher AH batteries remain at 12.6 volts.

[Chance]

Originally Posted by Tfryman

What you are saying is correct except for one thing: "Internal Resistance" really has no bearing on charge rate. Sure, internal resistance does impede both charging and discharging current and creates heat in the battery, but what determines the amps going into a battery is the difference between the source voltage (the charger) and the battery voltage. When the battery voltage increases, charging current will decrease. When charging batteries in parallel, as long as they have the same chemistry they will all have the same "full" battery voltage. Some of the batteries may reach that voltage before others, even if they all are the same make/model/AH/age. The ones that reach that voltage first will have almost no current applied to them, the lower voltage batteries will be receiving the charging current. When the charging circuit senses charging current to all batteries has decreased to its pre-set value, it will kick the charging voltage down to the "maintaining" voltage; normally about 13 volts.

Perhaps semantics, but if you have two batteries with different internal resistance such that one gets more current than the other, doesn't that mean faster charging -- everything else like charge voltage being the same?

An argument can be made that the battery with less resistance gets slightly higher current and therefore charges slightly faster. And if charging is interrupted prior to both batteries reaching 100% state of charge (say one at 70% and other at 60% as an example) then the two "should" equalize to a common voltage. Some "experts" claim this is inefficient, but I'm not sure it would be a deal breaker for me.

I would not discard a good battery (or two) just because it's a little different, unless they are so small that they don't contribute much anyway. That's why I'd confirm the capacity of the Dekas. Something doesn't seem right with the 75 A-hour rating.

[tinman01]

Originally Posted by Chance

Perhaps semantics, but if you have two batteries with different internal resistance such that one gets more current than the other, doesn't that mean faster charging -- everything else like charge voltage being the same?

An argument can be made that the battery with less resistance gets slightly higher current and therefore charges slightly faster. And if charging is interrupted prior to both batteries reaching 100% state of charge (say one at 70% and other at 60% as an example) then the two "should" equalize to a common voltage. Some "experts" claim this is inefficient, but I'm not sure it would be a deal breaker for me.

I would not discard a good battery (or two) just because it's a little different, unless they are so small that they don't contribute much anyway. That's why I'd confirm the capacity of the Dekas. Something doesn't seem right with the 75 A-hour rating.

The batteries are the Deka Promaster 75 amps @ 120 min

[Chance]

Originally Posted by tinman01

The batteries are the Deka Promaster 75 amps @ 120 min

Ok thanks, that explains a lot. Common rating for deep cycle batteries is based on 20-hour discharge rate, not 2 hours.

Still, if your battery is rated for 75 Amps for 2 hours, that's 150 Amp-hours, and that's at the tougher 2-hour rate. The 20-hour rate should be even higher, like around 200 Amp-hours or so.

[tinman01]

Originally Posted by Chance

Ok thanks, that explains a lot. Common rating for deep cycle batteries is based on 20-hour discharge rate, not 2 hours.

Still, if your battery is rated for 75 Amps for 2 hours, that's 150 Amp-hours, and that's at the tougher 2-hour rate. The 20-hour rate should be even higher, like around 200 Amp-hours or so.

I'm having a hard time wrapping my mind around the time frame they put on batteries. 75 amps for 2 hours how do they get 200 amps for 20 hours? Seems backwards. That seems to be on all the batteries, I just don't get how you can draw higher amps for longer times.

[16ACE27]

Originally Posted by tinman01

I'm having a hard time wrapping my mind around the time frame they put on batteries. 75 amps for 2 hours how do they get 200 amps for 20 hours? Seems backwards. That seems to be on all the batteries, I just don't get how you can draw higher amps for longer times.

It's not the time at the amp. The time is how long it takes to discharge a full battery. It is a 20 hour rate of current, a 2 hour rate of current. The AH capacity of most automotive batteries are calculated at the 20 hour rate as a standard. The faster you discharge a battery (higher current) the lower its actual capacity will be.

[TyCreek]

That Deka battery is, apples to apples, a 230ah battery. Basically the same as your new batteries. The 75 reference had me thinking small motorcycle size.

I don't know how old or degraded they are but if they're in decent working condition, really with FLA as long as you keep those paired they could be ok in parallel with the new ones. Monitor water consumption to know if the charge cycle is boiling some cells excessively.

[Chance]

Originally Posted by tinman01

I'm having a hard time wrapping my mind around the time frame they put on batteries. 75 amps for 2 hours how do they get 200 amps for 20 hours? Seems backwards. That seems to be on all the batteries, I just don't get how you can draw higher amps for longer times.

You're not following the "units" of measure correctly, which is what's confusing you.

A 200 Amp-hour battery (if rated at common 20-hour rate) would only deliver 10 Amps for up to 20 hours. Multiply current in Amps (10) times duration in hours (20) to get AmpXhours, or 200 A-hour.


Amp-hours is a poor attempt to rate energy capacity of batteries (must be careful because it doesn't include voltage rating). I think engineers would prefer to use kW-hours which takes voltage into account (notice that's how electric car batteries are rated).

[tinman01]

Originally Posted by TyCreek

That Deka battery is, apples to apples, a 230ah battery. Basically the same as your new batteries. The 75 reference had me thinking small motorcycle size.

I don't know how old or degraded they are but if they're in decent working condition, really with FLA as long as you keep those paired they could be ok in parallel with the new ones. Monitor water consumption to know if the charge cycle is boiling some cells excessively.

These were replaced by the dealer last Sept, so they are some what new, not much usage. Physically they are the same size as the Trojans but stated the 75 amp hour. So your saying that these are actually 230 amps about the same as the trojans? (Sorry to be so ignorant on this)

I started to go back to the dealer and just have them do it, but when they stated that they only use a 6 gage wire for everything! It really sent up a red flag! I may not know much but I do know you need a heavier battery cable than that. I used a 1 gage to connect the batteries together and will use a 4-0 to go to the inverter but I am thinking I need to go to a 2-0 if not heavier gage on the batteries and stay with the 4-0 to the inverter. between that and the price they wanted, not to mention the time frame they wanted quickly ruled them out!

[TyCreek]

If that Deka link I gave is your batteries... I'd call them the same. In parallel they should be ok since they're FLA. Again, "if" that link is a match and they're in good condition ... I'd think the added capacity is worth trying for awhile as it will likely make up for less than ideally matched parallel cells.

A series connection is where cell differences really battle in both the charge and discharge cycles. In parallel, FLA types are usually quite tolerant of some mismatch during the charge cycle. Just check for heat (especially for the first couple cycles) and learn your water consumption over the first few months or so to know how things are getting along.

There is such a thing as too big of wire or getting below internal resistance and that's not ideal. My rule of thumb that has worked well in projects, is size the series connection for max of the pair or a size bigger. Parallel output sized for length plus max total I might pull. Your 4K inverter in this case and it should give wire size recommendations in its specifications.