Renogy 100Ah Smart Battery Charge Levels.

[Ayjay]

I would be really grateful for some quick advice please - anybody who has read my posts about electrics will know they're not my strong point. My leisure set up is a Renogy 100Ah Smart Battery (about 17 months old) along with a CTek D250SE DC-DC Charger and Victron Bluesmart 240V Charger. TBH, I tend not to monitor charge levels of the battery too closely as it's usually plugged into the mains at home outlet or EHU when we're away - that said, it rarely exceeds 13.2V when plugged in.
We are in an AirBnB in Greece for a few days and the van is parked up without EHU. I checked the leisure battery late yesterday aftrernoon after it had been supplying the Dometic CRX50 for about 32 hours (and only the fridge). Although I confess that I didn't check the charge state when we arrived, it had just come of the back of 4 hours driving so I'm assuming it should have been good.

• The little plug in meter read 13.1V (but it does not give %).
• The BM2 App says 13.13V and 100%. See picture below
• The Renogy BT adapter says 13.2V and 31.87Ah / 31.87%. This is the one that's got me worried! See picture below.

I have to say that I thought 13.1V - 13.2V in a LifePo4 equated to at least 60 - 70%.

As a precaution, we took all the stuff out of the fridge and switched it off as I didn't really want to run the battery down too far BUT I'm wondering whether I've got a real problem or perhaps even the Renogy App readings might be incorrect (or the others). FWIW, the fridge was set at 'Level3' with the removable freezer compartment in place and everything is kept very cold (or frozen in the freezer). The fridge generally behaving as expected with no strange noises! That said, it has been very hot (well into the 30s) so the fridge is probably having to work quite hard.

 

[roadtripper]

If the BMS is tracking capacity purely from the cell voltages that can get out of sync if you have regular shallow charging cycles - that type of tracking can benefit from being run flat occasionally to calibrate.

So long as you have the means to give it a charge there's no immediate harm in risking the battery run flat. It's advised to keep LiFePo above 20% for maximum life but it's not nearly as critical as the 50% rule for lead acid.

 

[Bigsidavies]

I have exactly the same setup but with the addition of a 100w solar panel through a Victron mppt.
If it’s hot then the fridge will be working overtime and being down to 31% after a day and a half isn’t too surprising.
However, it is highly possible that you did not start with the LB at 100%. When the ctek charges the battery to 100% it stops charging and waits until the battery has dropped again before restarting the charge sequence. It doesn’t put charge in to compensate for useage.
So you may have arrived at your current location with maybe just over 80% in the LB. It’s happened to me and it’s quite annoying.
I got to Busfest last Thursday with 91% in the battery even though it had hit 100% earlier in the day.
Since I installed the seperate solar controller this is less of an issue as the Victron throws charge in. However Thursday was crap weather.
Also, why are you worrying about the LiFePo being at 31%? It’s fine for regular 80% dod and it doesn’t bother me if it was occasionally to go flat. Although I’ve only done that once. That was the whole point of LiFePo for me. If I need all the power then it’s there to be used.

 

[TallPaul_S]

13.1v is 40% and 13.2v is 70% for LiFePO4, if you started with 100%, 32hrs of 1.5Ah per hour fridge consumption is about 50Ah so I think you're about right on the voltage/percentage - you're at about 50-60%.

So the BMS on the battery is out of sync - can you see if it fully charged on your drive?

Ignore the bm2 percentage, that's quite obviously not accurate.

I'm not sure what method the Renogy battery uses for percentage but even a Renogy blog says to not rely on the percentage numbers!

 

[Ayjay]

Thanks for the inputs people. As mentioned above, I switched the fridge off - It will be going back on tomorrow morning before we do a 3-4 hour drive up to Patras and I will see how the charge looks then.
@Bigsidavies - I've also got a solar panel on the roof (but 160W) which I didn't mention as the van is parked completely in the shade so was irrelevant to this issue (or, at least, that was my thinking).
I think the main cause for my concern in all this was what seems to be an inconsistency between the reading on the Renogy BT App for Voltage and Ah%. @TallPaul_S pointed out,13.1V is around 40% and 13.2V is around 70% - The BT App for voltage reading was closer to the upper end with neither being near the 31.87% mark. This is the chart that I usually refer to which I think I found elsewhere here on T6F and which I turned iinto a fridge magnet (note: it reckons 13.1V is 60% though).

 

[Ayjay]

PS. I went out and had another look at it. Despite what I said about the solar panel being entirely in shade, it does get a bit of sun early in the morning. The BT app reading is now 41.03 Ah / % at 13.3v so I think something is definitely going on with inaccurate readings somewhere as those numbers don’t correlate. Sorry I didn’t think to screenshot it but I’ll have another look late on this afternoon.

 

[TallPaul_S]

This is from Renogy's own blog here Expectations vs. Reality: Making Sense of Battery Voltage & Percentages

 

[Ayjay]

@TallPaul_S. Thanks for that. The chart is particularly useful but with my limited understanding, I can't help thinking that the rest of it is just Renogy excusing themselves for not having a particular efficient management information and reporting system built into the battery which, given their reputation, I'm actually quite surprised and disappointed at. Correct me in the very likely event that i'm wrong but surely all the BT2 module is doing is reporting exactly the same basic information (ie. as supplied by the battery) as the 500A Battery Monitor would if that were installed instead (it's just that the 500A would be able to take and report a more detailed feed from the battery).
I've just taken another screenshot from DC Home attached. As can bee seen, the numbers still don't really correlate to the chart above, but I guess they're getting closer - as can be seen, it's now reporting 58.89% and 13.2V. I suspect the latter is accurate but according to the chart above, 13.2V should be 70%.

 

[TallPaul_S]

@TallPaul_S. Thanks for that. The chart is particularly useful but with my limited understanding, I can't help thinking that the rest of it is just Renogy excusing themselves for not having a particular efficient management information and reporting system built into the battery which, given their reputation, I'm actually quite surprised and disappointed at. Correct me in the very likely event that i'm wrong but surely all the BT2 module is doing is reporting exactly the same basic information (ie. as supplied by the battery) as the 500A Battery Monitor would if that were installed instead (it's just that the 500A would be able to take and report a more detailed feed from the battery).
I've just taken another screenshot from DC Home attached. As can bee seen, the numbers still don't really correlate to the chart above, but I guess they're getting closer - as can be seen, it's now reporting 58.89% and 13.2V. I suspect the latter is accurate but according to the chart above, 13.2V should be 70%.
View attachment 258764

Battery BMS's very commonly don't have the most accurate shunts, yours is probably the same.

My 230Ah fogstar BMS doesn't read any amp draw or charge of less than 1a, which means if you have a small load running constantly, the SOC is never accurate.

The 500a battery monitor, or any proper standalone shunt, will be able to see every single milliamp going in and out of the battery, and therefore accurately present a state of charge.

 

[Ayjay]

@TallPaul_S. Cheers for that - I'll have a look at the 500A when we eventually get home.

 

[roadtripper]

Small currents on a high current shunt are always going to be an issue - the higher current the shunt the worse it will be.

The reason is that a shunt is just a very highly engineered very low value resistor and the measurement is the very tiny drop in voltage across it.

If you are trying to measure 500A (while trying hard to not cause any energy loss through heating) then the resistance and voltage drops will be tiny - trying then to also measure 50mA (10000 times less) to the same accuracy is hard, you're in a regions where tiny stray induced voltages from other wiring and your phone etc will be of the same size as the voltages you are trying to measure.

BMS shunts main job is to monitor bulk charge and spot excessive discharge. Stuffed in the case with all the other BMS wiring they don't stand much chance, an external shunt stands much more.