Battery Mixing integrating Lithium Lifepo4 AGM or Gel

[Screen Saver]

Firstly let me say I to am not convinced one way or the other about the BMS, but I have yet to see a solid argument either way. I am learning thinking trialing, looking for reasons to go one way or the other.

My understanding of LifePo4’s is actually they are more tolerant of over voltage, ie Lifepo4 will deal with up about 16.8v and less damage will occur for the same amount of time. So I don’t think there is any information to support one technology is better than the other if you exceed recommended parameters. How you currently prevent this has no reason to be more unreliable as far as I can see and the resulting risks in the case of LifePo4 are the same.

As to the alternator protection yes perhaps in some cases, but that would not work easily in my case as there 4 charging systems on board and I would imagine most yachts have at least two, possibly 3. I also agree with the other comment it defeats its purpose. A manual change over is possibly two late and an automated change over with relays adds risks itself and one way diode, triac, or other such device also defeats its purpose.

As to the “problem with permanently connected cells” and short in one cell. This is pretty much in my view a risk we are already accepting. In my case I had 800amphs of Gell, so a short across that would still potential dump enough power into the short, to cause a fire. An internal short of one cell in either environment could do the same, and possibly worse in Gell or AGM as they are all contained in the same battery and share much of the internal environment. In the case of my LifePo4’s the runaway of one cell can not effect the next cell heat wise as there is a gap and each is in its own enclosure. Also in my case each battery ( or bank of 12v) has a at the moment a resettable 60amp fuse on it. This will be reduced shortly once a determine worst case load, to probably 50amps per 12v bank (150 total), so a short external or in one 12v bank can not drag more than 60amps out of the other. I have placed a short using 000 gauge to test and this did not trigger the BMS to shut down, it blew the fuse. The BMS switch's off after about 3 seconds on short from memory.







I wont comment on ABYC…..as you would guess I’m not supporter. Even the concept of this Im sure would be out of line and no doubt I would be showered with opposing views..

 

[Screen Saver]

So we are now a few months down the line and so thought I would just provide an update;

Everything at this point seems to be going pretty well in general but I am paying more attention to these batteries than either I have in the past or would like to. This is possibly due to the general public's thoughts on the dangers of Lithium, my bending the rules and my learning curve.

The blue tooth application that can monitor the Battery management system seems to struggle when there are two BMS units. For example I have a couple of old FAB's ( ie phone tablet) they have about a 6" screen use bugger all power, one runs my anchor watch, and BMS plus the FAB provides internet hot spot and the other monitors my second BMS unit. In my case it seems I have less than about a 10% chance of getting each FAB to monitor the respective BMS units at the same time. Either one will monitor either BMS, but try to get each of them to monitor the other and yea probably 1 in 10 it will work. So for now I monitor one set, and then the other.

Due to the above I am pondering changing the configuration from 2 x (4 cells in series to a bank) to 1 bank of 4 x (2 cells parallel) then I need only monitor one BMS. It would also help with balancing and results in spare BMS However if one of two cells in parallel had a problem that might not be obvious and I would need to down grade my charging system to 50 odd amps, rather than 50 across each BMS. Again not ideal. Its also conceivable this blue tooth problem could be mine due to the number of wireless devices.

Many of the default settings were very high and up above where I would typically allow them to run. I have a similar problem on my Ficher Panda Gen set for emergency shutdown. So in regards to my batteries I have reduced many settings in order to quickly react, Things like pack over voltage, and cell over voltage, and cell over current. I will or should never be over 150amps either charge or discharge and the default was in the order of 800amps discharge, same concept applies to voltage and temperature. Over Temperature was set to 50c its now set to 40c for example.

Because the original Gells at rest want to sit at around 12.9 and Lithium wants to sit around 13.1 13.2 at night there is a continuous current flow of about 0.8 to 1.8amps flowing from Lithium to Gell (more when Lithium is over 13.2) in order to hold the Gells up. Although this amounts to only about 10 -12 amp hrs it is not ideal and you still need to put it back. There are a number of isolation or auto switching solutions and I am still pondering which will work better in my case, or for simplicity reasons perhaps I should just accept the drain. Obviously when the Lithium gets down to the same as the Gells this ceases

With non-lithium technology you can get away with watching the voltage as a very rough gauge as to the state of your batteries (not recommended by any means). But with Lithium its far more important that you pay attention to actual amphrs or watts consumed. For example you may have watched a couple of movies on the big screen on your original batteries and it is typically quite clear on non Lithium even by voltage that yep you have consumed say 50% of your capacity. On lithium perhaps you have watched 4 episodes of your favorite series and at first glance it may not have even made a dent in the voltage which might still be around 13. The result could be you need to put back some 150amphrs the next day or you have consumed 60% of your capacity. So in short it’s easy to consume more than you realize or can put back so you must be able to monitor Amphrs or Watts Having a large capacity is all well and good, but you need to figure out how you can put it back.

As of a couple of days ago I disconnected my house Gells, so 1 gell remains which is 7 years old as a start battery for main engine and generator. The main reason I did this is ultimately I will have only LifePo4 for the house batteries, but in the interim if I should have problems I didnt want to lose the house supply or damage my alternator

 

[Maine Sail]

Because the original Gells at rest want to sit at around 12.9 and Lithium wants to sit around 13.1 13.2 at night there is a continuous current flow of about 0.8 to 1.8amps flowing from Lithium to Gell (more when Lithium is over 13.2) in order to hold the Gells up. Although this amounts to only about 10 -12 amp hrs it is not ideal and you still need to put it back. There are a number of isolation or auto switching solutions and I am still pondering which will work better in my case, or for simplicity reasons perhaps I should just accept the drain. Obviously when the Lithium gets down to the same as the Gells this ceases

A lot of folks underestimate how much energy an LA battery can bleed off over 24 hours. This parasitic load needs to be stacked onto the contactors load, if using contactors for BMS protection.

The easiest way to parallel the GEL and LFP, when charging with the alt, is to simply use a relay that is capable of handling the charge current. You then excite the relay coil via the ignition run position. Now the LFP bank is in parallel with the GEL, only when charging via the alternator. Shut down the engine, turn off the key and the batteries are no longer in parallel. Another option is a FET based isolator but they are a lot more expensive than a relay/solenoid. Typical VSR/ACR/Combiners are built for lead acid combine/uncombine voltages. Some are custom programable but all you're doing is trying to protect the alt and any heavy duty relay can be used for this.

 

[bbrowncods]

There is a guy on FB that discusses this (mixing battery types, not oil filters) on his sailboat.

Think of it this way. Li is the most sensitive, easily damaged and expensive. Therefore set the charging parameters to what the Li batteries need. Also set the low voltage cut out to what the Li batteries require per manufacturer.
Now the Lead Acid batteries can be connected. Only do so when equal voltage! They will act as a load when less than the voltage of the Li batteries, and act as a small charger when voltage is higher than the Li. In either case the Lead batteries will fall well within the voltage parameters of the Lithium. The Lead batteries are less expensive (relatively) than the Li and more tolerant of less than optimal charging. But the advantage to having Li tied in is that it minimizes the depth of discharge of the Lead, thereby extending their life. If you are charging with multiple chargers (alt, solar, inverter/charger) then set them the same. If you can't set the alternator then it will charge to the Lead Acid voltages, and the BMS will protect the Li batteries.
Now here comes the problem. When, or if, the BMS separates the Li battery from the system, for whatever reason, you now have two sets of batteries with different voltages. When the BMS reconnects them, that sets up high current situation due to the differences in internal resistance between the battery types. The BMS will more than likely cut off the overcurrent, but if that fails you are in trouble. So I would place a power resistor to limit the current between the banks, as well as a circuit breaker - a double backup to the BMS. No different than protection between a normal load, in this case the Lead batteries, and the power source, the Li batteries.

 

[dlochner]

There is a guy on FB that discusses this (mixing battery types, not oil filters) on his sailboat.

Think of it this way. Li is the most sensitive, easily damaged and expensive. Therefore set the charging parameters to what the Li batteries need. Also set the low voltage cut out to what the Li batteries require per manufacturer.
Now the Lead Acid batteries can be connected. Only do so when equal voltage! They will act as a load when less than the voltage of the Li batteries, and act as a small charger when voltage is higher than the Li. In either case the Lead batteries will fall well within the voltage parameters of the Lithium. The Lead batteries are less expensive (relatively) than the Li and more tolerant of less than optimal charging. But the advantage to having Li tied in is that it minimizes the depth of discharge of the Lead, thereby extending their life. If you are charging with multiple chargers (alt, solar, inverter/charger) then set them the same. If you can't set the alternator then it will charge to the Lead Acid voltages, and the BMS will protect the Li batteries.
Now here comes the problem. When, or if, the BMS separates the Li battery from the system, for whatever reason, you now have two sets of batteries with different voltages. When the BMS reconnects them, that sets up high current situation due to the differences in internal resistance between the battery types. The BMS will more than likely cut off the overcurrent, but if that fails you are in trouble. So I would place a power resistor to limit the current between the banks, as well as a circuit breaker - a double backup to the BMS. No different than protection between a normal load, in this case the Lead batteries, and the power source, the Li batteries.

While I have no doubt there is a way to connect LiFeP batteries and FLA or AGM batteries, I think the big question is do you want to be on the bleeding edge of technology or leading edge?

I remember when AGMs and Gel batteries were the bleeding edge.

 

[heritage]

I chose to isolate the small agm battery and large lifepo4 battery bank with a 20 amp dc to dc charger and is working fine on my boat.

I just saw a video that had some compelling reasons for combining the banks. The video is rather long but well done. He doesn't address the possibility of a shorted cell in the fla bank. Although, I would think a fuse would stop a catastrophic dump of energy from the lifepo4 bank.

 

[Hayden Watson]

Please correct me if I am wrong but the main advantage of this method is to prevent alternator damage if the BMS disconnects the LFP will the alternator is charging. On my boat, the house and start are completely separated. The only connection is made when charging through an ACR with the regulator sense wire on the input side. It seems that with this system, I set the external regulator to the voltage needed by the LFP with minimum absorption time and forget it. The BMS can shut off the LFP but there will be no spike because the start battery will remain connected and the regulator will control the voltage to it.

 

[heritage]

The advantage of the system shown in the video is that the Lifepo4 battery charges the Fla battery bank at night and keeps it from going below 50% soc, which prolongs it's life. I think it would be beneficial to those that already have a fairly large Fla battery bank that's having trouble staying above 50% by morning. This assumes that your primary source of recharge is solar.