Lithium 80% DOD rule

[markopolo-ClassB]

I typed lithium and rule for the title. It's more like lifep04 and guideline but that seemed boring!

How do you know if your lifep04 batteries are at or near 80% discharged?

The answer seems to be that you don't know, can't know and won't know unless you monitor state of charge (SOC).

These posts prompted me to do some Googling.
http://www.classbforum.com/forums/f5/hymer-aktiv-5333-49.html#post49426
http://www.classbforum.com/forums/f5/hymer-aktiv-5333-50.html#post49469

A coulomb counter "battery fuel gauge" type device would seem to ideal for triggering lifep04 recharging. (I think that's how Advanced RV does it in their RV's)

Take a 400ah battery bank, set the recharge request at 80% DOD (320 net Ah out). Has to be net of other charging inputs. As soon as the net total ah out exceeds 320 then the unit triggers the engine or other type generator to start. You could have unlimited trigger points such as 50% SOC or 70% SOC - let the end user choose.

You can find several charts showing lifep04 capacity mostly depleted at much higher than expected voltages.

Here's one from TI: ( http://www.ti.com/lit/an/slyt402/slyt402.pdf )



Looks like approx 90% of the capacity of that battery is gone but at 12.72V no load.

Here's one from GWL Power: ( lithium & solar power LiFePO4, The voltage levels and the capacity of LFP cells... )



86% of the capacity used up but still 12.64V with 0.4C load. (40ah load on a 100ah battery)

This graph shows that at a 1.5C rate of discharge a dramatic fall off (think cliff!) starts at or before 12.2V (lithium & solar power LiFePO4, Restoring a partly discharged cell A customer... )



It would appear that, if triggering a recharge based on voltage, then you'd have to know both the load (rate of discharge) and the capacity of the battery bank and set the voltage trigger point accordingly as it will vary quite a bit as shown above. I don't see a one size fits all type solution with a voltage trigger but it does appear that a SOC based recharge trigger would work in all of the situations shown above.

 

[booster]

X10---It continues to baffle me why the manufacturers build these fancy systems and tout their technical expertise, but ignore what is really important. The conclusion has to be that they want the customers to be unable to tell if the system is as great as touted. I guess you could compare it to smart charger manufacturers that tell the customers they will always charge the batteries perfectly, when we now know that most of them don't come even close to doing what they claim.

 

[gregmchugh]

I agree that using a shunt to measure net amps in/out of the battery and keeping track of state of charge by integrating net amps provides the most straightforward method for getting an accurate state of charge estimate. And using this state of charge estimate to trigger generator autostart is probably the only reliable method to trigger the autostart at a specific state of charge. A voltage measurement of the battery bank provides a very inaccurate measure of state of charge for the reasons you mentioned.

The modular design of the Ecotrek battery system presents a challenge using this method since a single shunt on the battery bank is pretty useless. To get an accurate measure of net amps in/out of the battery cells requires a shunt in each Ecotrek module to compute the state of charge for each module and a way to collect this info in a central location to compute the overall state of charge of the battery bank. This can be implemented but not as easy to do as with the integrated design where all the battery cells are connected directly to create the battery bank.

If the design goals of the autostart system are to allow an unattended van to run AC for as long as possible to protect pets or to use AC overnight without attention I think that a simple voltage triggered autostart system can be configured to reliably meet these goals. If the voltage trigger is set high enough to insure the Ecotrek modules never shut down due to low state of charge during Voltstart operation and set low enough to insure that none of the energy available from the engine run periods is lost trying to add amps to a full battery bank then all of the potential energy will be available to run the AC. The maximum energy available is a combination of 80% of battery bank capacity plus the energy generated by 5 X 35 minutes of engine generator operation (or whatever the limits are on engine generator starts and run times).

Is this an elegant solution, maybe not, but it is simple to implement and seems to me that it provides the same performance in these two scenarios as you would obtain from an autostart system using a state of charge trigger.

 

[mojoman]

Each lithium battery cell model has its own performance profile provided by the MFG with the spec sheet. They are not all the same. Even the total AH are usually under rated as a 100AH cell will usually hold 5-10% more AH than rated. If you are taking them down to 80% DOD often, then you should add more cells, the pack will last longer. At 900AH, I have much more than I would normally need but I have no worries about SOC other than keeping them under 80% while in storage. My solar panels will charge them to 100% if I don't monitor while they are turned on.

 

[gregmchugh]

X10---It continues to baffle me why the manufacturers build these fancy systems and tout their technical expertise, but ignore what is really important. The conclusion has to be that they want the customers to be unable to tell if the system is as great as touted. I guess you could compare it to smart charger manufacturers that tell the customers they will always charge the batteries perfectly, when we now know that most of them don't come even close to doing what they claim.

My conclusion is a little less nefarious, I think they felt that the time, resources, and money spent on providing detailed system data would be wasted on giving the majority of their customers, who have no interest in the data, a feature that would never be used. This allowed them to get the system to market faster, clearly too fast to get it ready for prime time, but faster than if they did include system status displays.

For the minority of customers who would value system status info, I guess they need to be convinced that providing this additional feature for these customers is a wise allocation of the always limited time, resources, and money available for product development. I guess I have yet to see a significant demand for this feature except from a small percentage of buyers who are over represented here on this forum. Would I demand one, sure I would. If they do make it available, I would expect a hefty price tag for the option, rather than making a standard feature. But, I could be wrong...

 

[markopolo-ClassB]

I agree that using a shunt to measure net amps in/out of the battery and keeping track of state of charge by integrating net amps provides the most straightforward method for getting an accurate state of charge estimate. And using this state of charge estimate to trigger generator autostart is probably the only reliable method to trigger the autostart at a specific state of charge. A voltage measurement of the battery bank provides a very inaccurate measure of state of charge for the reasons you mentioned.

The modular design of the Ecotrek battery system presents a challenge using this method since a single shunt on the battery bank is pretty useless. To get an accurate measure of net amps in/out of the battery cells requires a shunt in each Ecotrek module to compute the state of charge for each module and a way to collect this info in a central location to compute the overall state of charge of the battery bank. This can be implemented but not as easy to do as with the integrated design where all the battery cells are connected directly to create the battery bank.

If the design goals of the autostart system are to allow an unattended van to run AC for as long as possible to protect pets or to use AC overnight without attention I think that a simple voltage triggered autostart system can be configured to reliably meet these goals. If the voltage trigger is set high enough to insure the Ecotrek modules never shut down due to low state of charge during Voltstart operation and set low enough to insure that none of the energy available from the engine run periods is lost trying to add amps to a full battery bank then all of the potential energy will be available to run the AC. The maximum energy available is a combination of 80% of battery bank capacity plus the energy generated by 5 X 35 minutes of engine generator operation (or whatever the limits are on engine generator starts and run times).

Is this an elegant solution, maybe not, but it is simple to implement and seems to me that it provides the same performance in these two scenarios as you would obtain from an autostart system using a state of charge trigger.

A voltage triggered charging request would extend runtime on batteries and prevent a complete discharge. That's about all I can think of as being fairly certain.

I don't see how you'd ever know the SOC though other than after being plugged in for a long time. You could assume 100% SOC then.

If you don't monitor state of charge then:

1. How would you know what the SOC was at the start of an A/C run?
2. How would you know what the SOC was at the start of charging?
3. How would you know what the SOC was at the end of short duration charging?

If the a/c (as an example) continued to run after an engine start what is the net ah input? 50ah? 70ah?

You'd could only guess at SOC and could be off by 100's of ah if the battery bank was large enough.

 

[gregmchugh]

Each lithium battery cell model has its own performance profile provided by the MFG with the spec sheet. They are not all the same. Even the total AH are usually under rated as a 100AH cell will usually hold 5-10% more AH than rated. If you are taking them down to 80% DOD often, then you should add more cells, the pack will last longer. At 900AH, I have much more than I would normally need but I have no worries about SOC other than keeping them under 80% while in storage. My solar panels will charge them to 100% if I don't monitor while they are turned on.

Is it clear how much the battery life is lost by discharging to 80% vs other levels? Is there a curve similar to those published for AGM batteries? Is the curve similar to the AGM curve with a fairly smooth degradation of cycles vs discharge or does the life drop rapidly at some point?

Roadtrek is providing a 6 year warranty and it does appear they backed off from the original plan to discharge down to 10% and are now at 20%.

Many of us older folks may not be alive long enough to wear out lithiums...

 

[booster]

Your point might be more realistic, at least when you compare to normal smart chargers, if they didn't include ridiculously inaccurate idiot lights to indicate how much SOC the customer has left. It only makes sense that if you are putting in a charger that chronically under or over charges, you don't want to put in a monitor that will show that weakness to the customer. I truly believe this is a systemic attitude throughout the RV industry. Even abused batteries are going to make it through most warranty periods, so there is very little incentive to do a better job. Bottom line is that you can get away with less than stellar product, if you don't give the customer any way to know the product is not all that great. It happens all the time, in almost all types of products, so we all just need to allow for it.

 

[gerrym51]

the answer is Roadtrek does not want you to know

 

[booster]

Is it clear how much the battery life is lost by discharging to 80% vs other levels? Is there a curve similar to those published for AGM batteries? Is the curve similar to the AGM curve with a fairly smooth degradation of cycles vs discharge or does the life drop rapidly at some point?

Roadtrek is providing a 6 year warranty and it does appear they backed off from the original plan to discharge down to 10% and are now at 20%.

Many of us older folks may not be alive long enough to wear out lithiums...

There actually was a life cycle vs DOD posted in the one of the battery life threads that showed lithium to be even more susceptible to deep discharge damage than lead acid wet or AGM batteries.

 

[gregmchugh]

A voltage triggered charging request would extend runtime on batteries and prevent a complete discharge. That's about all I can think of as being fairly certain.

I don't see how you'd ever know the SOC though other than after being plugged in for a long time. You could assume 100% SOC then.

If you don't monitor state of charge then:

1. How would you know what the SOC was at the start of an A/C run?
2. How would you know what the SOC was at the start of charging?
3. How would you know what the SOC was at the end of short duration charging?

If the a/c (as an example) continued to run after an engine start what is the net ah input? 50ah? 70ah?

You'd could only guess at SOC and could be off by 100's of ah if the battery bank was large enough.

On a Roadtrek you have no accurate measure of state of charge displayed by the system in the current implementation, all you get is a battery voltage reading taken at the output of the overall battery bank. They give a voltage value to shoot for when storing the vehicle but I don't recall any numbers to use for full charge or any other charge levels.

The BMS in each Ecotrek may or may not track state of charge for that module, who knows. They have some method to disconnect the loads at 80% discharge and some method to disconnect the chargers at full charge. Is this based on state of charge or individual cell voltages or something else? Again, they don't reveal that level of info about the design...

After the final Voltstart cycle the system will continue providing power until all the Ecotrek modules shutdown as the 80% discharge point is reached in each module.

 

[mojoman]

They probably have the same safeguard my cheap mini BMS has, a low cell voltage trip. Mine will alarm first then tripp the main battery breaker around 20%SOC, temp compensated.

 

[markopolo-ClassB]

Interesting LiFePO4
SOC chart from Shorai - Shorai FAQ

This is very different than what we are used to seeing for lead acid batteries.

and another lifep04 SOC chart here: Days of Wine and Roses - EVTV Motor Verks

Again, much higher voltages than you'd expect if only familiar with lead acid batteries. 20% SOC = 12.89V (3.222v x 4 cells)

 

[booster]

Very interesting. Also hardly any voltage difference between 90% and 30%, something like a tenth and a half of a volt. Also note that it has to be no load voltage, although no note about resting so maybe not needed.

It really does make one wonder how you would be able to determine when to start the van engine based on voltage, especially if you have 100 amps of air conditioner running, maybe cycling, which would probably severely affect the voltage readings.

 

[markopolo-ClassB]

I don't know how you could either. It is very much dependent on (1) the size of the battery bank & (2) the rate of discharge.

It's worth re-posting this image from the first post in this topic.

0.4C discharge rate (40ah), 100ah battery - I think you could multiply that by 4 to get a 160ah discharge rate on a 400ah battery bank, it's the same 0.4C rate. That would run an a/c no problem. The chart indicates that you should start recharging at around 12.64V to avoid going below 10% SOC.

 

[gregmchugh]

These curves show discharge under loads for cells similar to the ones used by Roadtrek and ARV...


http://elitepowersolutions.com/docs/Cell Charging and Discharging Curves.pdf

 

[markopolo-ClassB]

That confirms it also.

12.8v or so looks to be around 75% discharged at 0.5C (50A) to 1C (100A) discharge rate.

 

[gregmchugh]

That confirms it also.

12.8v or so looks to be around 75% discharged at 0.5C (50A) to 1C (100A) discharge rate.

Roadtrek and ARV use the 200AH version of these so I think in their case the curves would be for 100 amps and 200 amps assuming the curves are typical of different size cells...

 

[Davydd]

X10---It continues to baffle me why the manufacturers build these fancy systems and tout their technical expertise, but ignore what is really important. The conclusion has to be that they want the customers to be unable to tell if the system is as great as touted. I guess you could compare it to smart charger manufacturers that tell the customers they will always charge the batteries perfectly, when we now know that most of them don't come even close to doing what they claim.

I'm assuming you are talking about only one Class B manufacturer in that being Roadtrek since with Advanced RV I get rather detailed readout information down to each individual battery cell for voltage and temperature and have a lot of programmable control on how I want it to charge with Autogen.

For starters I have 16 200ah 3.2v LiFeP04 cells with four in series to make a nominal 12v which is actually closer to 13.8v fully charged. Four in parallel give me the nominal 800ah published. When the the battery bank uses 640ah (80% DOD) then the bank automatically shuts down completely. I then have to manually restart it by physically holding down a toggle switch with shore power plugged in or the engine running for up to 6 minutes or 1% SOC before the batteries reconnect.

As for Autogen, it is completely different than Voltstart. I can program when I want it to start and how long I want it to run in minute increments up to 2 hours each time it starts. Like Roadtrek it will automatically start 5 times. Then to reset, one has to start the engine with the key. In theory one can idle the engine up to 10 hours. I can also program the hours when it can auto start so I can lock out campground quiet hours. I can also program at what state of charge I want Autogen to come on so I don't come close to that 80% shutdown. Right now I have it set so it should come on at about 50% SOC and I rarely come close to that. That means if I am down 400ah and my Autogen is charging at a rate of well over 230 amps with high idle I would not run a full two hours (currently set at 1 hr 55 min). When the batteries fully charge, Autogen shuts off before the preset maximum time.

The Silverleaf screen shows net charge or discharge in its readout. So, if I am charging at say shore power of about 110 amps and have an average at rest (inverter and refrigerator on and all the other standbys) of about a 10 amp discharge, the screen would show 100 amps input. Those are just generalized numbers for example purposes. The numbers constantly fluctuate but I get a good reading of what's going on when I run a microwave, coffee maker or induction cooktop for instance. During the day solar is also charging. After a while you get a sense of what is going on.

There are no secrets or diversions. The information is as accurate as the installation can monitor.

Here is the individual cell battery status. You can also see the heat pads are triggered (green ends) and the heat pads (flame symbol) are actually heating the batteries to keep them above 41 deg. F.

This is the DC Power screen that shows charging coming from shore power and the SOC almost complete. The difference between 112a charge and net 90a means there was a draw off the batteries as I mentioned at that particular time. At the left of the battery symbol is the average battery bank temperature. The bottom number is the SOC. The voltage number will drop with the SOC. That voltage number has read as high as 14.2v when charging.

I don't pretend to know how all this works or how measurements are taken but after almost two years I have a good sense as to what is going on and how I can manage an extended stay boondocking. When I arrive at a campground now I know I can boondock easily for two days straight and run my van with the inverter always on indiscriminately cooking, making coffee, running fans, etc. without thinking about battery conservation and not having Autogen come on. Other than attending rallies and such events we rarely sit more than 2 days without driving somewhere for some reason. I'm boondocking now at about a 95% rate. I was in Grand Marais, MN last week for two nights and did not bother to plug in.

As for air conditioning you know I rarely use it as I have said numerous times so it is not a concern for me. I do know one Advanced RV owner claimed to go overnight 13 hours with his air conditioning set to on with his 800ah battery bank and not have Autogen come on. Obviously the air conditioning was not running continuously. If that is true I suspect I could easily get through a day and would never encounter 5 auto starts. Besides, one would have to drive somewhere anyway and refill the diesel tank eventually and you would have to be pretty dumb to go through 5 auto starts with all that detailed information at hand.

 

[markopolo-ClassB]

Would you mind posting the voltage along with the SOC % at various levels of discharge? That bit of data could be very useful to those who don't have the level of monitoring that you do and also interesting to those of us with lead acid batteries.