Battery mixed marriage, anyone?

A sincere question that hopefully doesn't spark a debate

Background:
My build will be electrically dependent.
It will be spilt bus
It will be, at least a two alt, single battery system. (Z101-ish)
Strongly considering a two alt, two battery system. (Z14-ish). Either would have a dash of Froehlich-ness added.

The weight advantage of EarthX types make it very temping to go dual, semi-redundant. This would probably pay for itself in time as I don't anticipate buying expensive component backup (e.g. EFIS) batteries.

The SPF mode introduced via the BMS and lessened charge fault tolerance (high V) is a concern for me with LiFe. Was considering having the bus being directly fed by the 20 amp secondary alt tied to an EarthX. The other would be an AGM lead acid type.

Short question. Anyone have mixed battery types on such setups? Experiences good/bad? Am I missing something?

I need to start laying out the physical equipment. Your experiences and insight would be appreciated.

If you are going to use batteries of differing chemistries you risk discharging one by it trying to charge the other when no alternator is running. It would probably be wise to have contactors for each and a procedure to insure that they aren't both on the buss at the same time without the alternator charging. They should be able to co-exist while charging.

Ed Holyoke

Freemasm said:

A sincere question that hopefully doesn't spark a debate

Background:
My build will be electrically dependent.
It will be spilt bus
It will be, at least a two alt, single battery system. (Z101-ish)
Strongly considering a two alt, two battery system. (Z14-ish). Either would have a dash of Froehlich-ness added.

The weight advantage of EarthX types make it very temping to go dual, semi-redundant. This would probably pay for itself in time as I don't anticipate buying expensive component backup (e.g. EFIS) batteries.

The SPF mode introduced via the BMS and lessened charge fault tolerance (high V) is a concern for me with LiFe. Was considering having the bus being directly fed by the 20 amp secondary alt tied to an EarthX. The other would be an AGM lead acid type.

Short question. Anyone have mixed battery types on such setups? Experiences good/bad? Am I missing something?

I need to start laying out the physical equipment. Your experiences and insight would be appreciated.

Click to expand...

I have a project underway that uses Z-14. I plan to use IDENTICAL batteries (EarthX) for this effort.

My RV8 and RV6 now have ONE alternator and a "large", plus a "small" EarthX. Initially the RV6 dual battery setup had a "small" AGM.

There are some issues with tieing together two batteries of difference sizes as well as different chemistries as mentioned above. So, if you are going to have different sizes AND if they are EarthX, I think they would want you to use their "auxiliary/backup" battery as the small one as it has additional protection. (Speak to Kathy for the final facts on EarthX).

My comments come from "experiments" with the EarthX, along with the follow-up dialogs had with Kathy and Reg.

jclark said:

SNIP…
There are some issues with tieing together two batteries of difference sizes as well as different chemistries as mentioned above. So, if you are going to have different sizes AND if they are EarthX ….SNIP

Click to expand...

While I agree the different chemistries between EarthX and AGM is an issue, I disagree battery size difference is a concern. Each battery will draw current based on terminal voltage. Terminal voltage is a function of the output characteristics of the alternator.

The issue with different chemistries is associated with battery charging specifications. Mixing chemistries means you will compromise between the AGM and the higher charging voltage of the EarthX.

Note - If you end up with damaged batteries (ie run it into the ground) I recommend replacing it. While I have not done the research on EarthX I could envision weird stuff happening after a jump start.

For me, I find two PC-625 batteries to be a firm foundation for any IFR RV.

Carl

EarthX

I am running a modified Z-14 system with two ETX900s.

It works great.

To address your SPF of the BMS, look at how it is supposed to work. If you are using a B&C voltage regulator, Crowbar OV protection is built in; depending on which version you have, that crowbar should trip the alternator field at 16 volts +/- 0.2 volts. This should occur in about 40 miiliseconds or so. The BMS will take the battery offline at 16+/- volts plus 1 to 2 seconds (depending where you look). If you go by the numbers, the crowbar should always trip the field before the BMS even comes into play. The devil, as they say, is in the details. If the OV protection is on the high side, ie 16.2 volts and the BMS is on the low side, say 16 volts, then the BMS could take the battery offline before the crowbar trips the field...and then bad thing can happen. I know of only one case where this has happened, if there are others, chime in.

The are many people running this system that have had zero issues. It would seem that the solution might be to calibrate the OV crowbar to a lower voltage, say 15.8 +/- .2. This would guarantee that the crowbar trips the field prior to the BMS becoming involved. I have asked B&C if this would be possible. When I get an answer, I will post it.

Carl Froehlich said:

While I agree the different chemistries between EarthX and AGM is an issue, I disagree battery size difference is a concern. Each battery will draw current based on terminal voltage. Terminal voltage is a function of the output characteristics of the alternator.

<<SNIP, SNIP>>

For me, I find two PC-625 batteries to be a firm foundation for any IFR RV.

Carl

Click to expand...

Carl,

I think in general you are correct, BUT ....

The specific condition, I base the comment on is if you have a LARGE, fully charged EarthX and a relatively small almost fully depleted EarthX, and if you have one alternator and a setup where you basically "charge" the smaller backup battery from the larger, (diodes, switches, etc.) then the large battery can potentially dump large amounts of current to the little guy or at least more than he can handle in a short period of time without proper regulation.

We theorized that that is what happened to me in one of the test configurations.

Note that they recommend limits on the size alternator associated with certain batteries.

jclark said:

Carl,

I think in general you are correct, BUT ....

The specific condition, I base the comment on is if you have a LARGE, fully charged EarthX and a relatively small almost fully depleted EarthX, and if you have one alternator and a setup where you basically "charge" the smaller backup battery from the larger, (diodes, switches, etc.) then the large battery can potentially dump large amounts of current to the little guy or at least more than he can handle in a short period of time without proper regulation.

We theorized that that is what happened to me in one of the test configurations.

Note that they recommend limits on the size alternator associated with certain batteries.

Click to expand...

I do not disagree, I just do not have EarthX data to confirm your scenario.

But - for me if I ever were to use a couple of EarthX batteries they would be the same size (for various reasons other than this example), and I would not tolerate operation with one in such a state of discharge. For EarthX I suggest the need for such procedural protections.

Carl

Much thanks to all. Still have some questions but I'll reach out via PM.

rocketman1988 said:

I am running a modified Z-14 system with two ETX900s.

It works great.

To address your SPF of the BMS, look at how it is supposed to work...

Click to expand...

I'm not debating, I'm going off anecdote (not my style) but I have no direct data or experience. The OV scenario you mentioned is just one example. Have heard here and other places that the subject BMS(s) have failed and rendered the associated battery in-op. Have also heard where where the BMS have taken the battery off-line to protect the battery.

I'm fully aware of the plethora of variables associated with the installation/elect system architecture/operation/operator (pilot) system knowledge/etc. I can't gage the contributions of each of the aforementioned to a failure.

The BMS does add failure modes and in itself is a potential SPF. That fact isn't debatable. Just trying to understand and plan/manage any associated risks.

I want the technology to succeed. Just picking one up leaves an impression; almost feels like an empty "display" article. As mentioned, I'll have more questions but right now I'm working in circles; have to plan for physical layout of firewall stuff. Thanks again.

Cheers boys

It's been a while since I worked with batteries (RV and marine electrical systems was my thing for years). If I recall when a multi battery bank is connected in parallel the batteries will only take as much charge as the "weakest" link in the chain. So if you have 2 batteries of different sizes connected together and being charged. the larges battery will not store any more that the capacity of the smaller one.

I can for certain confirm mixing of different chemistry is a big no no.

YMMV.....

Freemasm said:

Much thanks to all. Still have some questions but I'll reach out via PM.



I'm not debating, I'm going off anecdote (not my style) but I have no direct data or experience. The OV scenario you mentioned is just one example. Have heard here and other places that the subject BMS(s) have failed and rendered the associated battery in-op. Have also heard where where the BMS have taken the battery off-line to protect the battery.

I'm fully aware of the plethora of variables associated with the installation/elect system architecture/operation/operator (pilot) system knowledge/etc. I can't gage the contributions of each of the aforementioned to a failure.

The BMS does add failure modes and in itself is a potential SPF. That fact isn't debatable. Just trying to understand and plan/manage any associated risks.

I want the technology to succeed. Just picking one up leaves an impression; almost feels like an empty "display" article. As mentioned, I'll have more questions but right now I'm working in circles; have to plan for physical layout of firewall stuff. Thanks again.

Cheers boys

Click to expand...

Do you have a specific thread referenced for the BMS failures? Just interested as I have two to contend with…

I don't and I apologize. As mentioned, I tend to ignore anecdote/tribal knowledge/etc. I'm an engineer as well; data and facts rule. I'm just beginning to geek up on this related electrical. It wasn't a single thread but info collected from here and other places. Some came from unnamed but well respected people in this thread. It's quickly obvious who you can trust here.

For a well engineered 2x2 electrical system, I'd be fine with a dual LiFe approach. I'll fly with you anytime in any conditions. For a 2x1, I'd probably plan around a single AGM. Simple componentry would be better in that config from an FMEA "modes" standpoint, IMO.

One thing does concern me as I look at the related, potential architecture (Z-14). If you need to isolate the aux battery for whatever reason, you take down the aux battery bus (second ign and second fuel pump). The "effects" side of the FMEA seems unreasonably high. If I'm seeing it wrong, please educate me.

Freemasm said:

I don't and I apologize. As mentioned, I tend to ignore anecdote/tribal knowledge/etc. I'm an engineer as well; data and facts rule. I'm just beginning to geek up on this related electrical. It wasn't a single thread but info collected from here and other places. Some came from unnamed but well respected people in this thread. It's quickly obvious who you can trust here.

For a well engineered 2x2 electrical system, I'd be fine with a dual LiFe approach. I'll fly with you anytime in any conditions. For a 2x1, I'd probably plan around a single AGM. Simple componentry would be better in that config from an FMEA "modes" standpoint, IMO.

One thing does concern me as I look at the related, potential architecture (Z-14). If you need to isolate the aux battery for whatever reason, you take down the aux battery bus (second ign and second fuel pump). The "effects" side of the FMEA seems unreasonably high. If I'm seeing it wrong, please educate me.

Click to expand...

The short story is I am running a modified Z-14; there is a diode bridge in my system and I am actually running both alternators all the time. The second alternator isn’t used as a standby. If you want to talk about it, send me a PM. I would be happy to talk about what I did…

Carl Froehlich said:

I do not disagree, I just do not have EarthX data to confirm your scenario.

But - for me if I ever were to use a couple of EarthX batteries they would be the same size (for various reasons other than this example), and I would not tolerate operation with one in such a state of discharge. For EarthX I suggest the need for such procedural protections.

Carl

Click to expand...

I concur.
And they have. In several of the batteries, if not all by now.

I just know that my “out of the ordinary or expected” use case (or test if one prefers) yielded results that they looked into and later recommended what “different” combinations were best.

My “tests” created warpage and weakness in the batteries in a way that was not expected. They got pictures and data and went to work. Much discussion with the chief engineer and thus more protection was added.

For those reading this far, this was YEARS ago.