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helpless asked

SOC on lithium batteries not increasing enough

I have 290w solar panels connected in parallel on the roof of my motorhome charging two 90ah lithium batteries through a 100/30 victron mppt.

I am not impressed with the extent to which the battery app is telling me that the SOC is recovering after what I consider is a fair day’s use age and sunshine (for the U.K.).

My expectation was that the batteries would receive bulk charge until nearly full then switch to absorption then finally float.

My experience is that with the batteries down to 50% SOC after three days off grid that by about 9.30 am the set up has switched to absorption and by 12.00 to float preventing the SOC from having any meaningful gain during the day.

Advice I received suggests that the victron mppt is too far from the batteries and I am planning to have it moved but is that the solution? The dealer measured the current at the victron output and again at the battery and no meaningful current drop (only at relatively low charge rate and the suggestion is I am losing out on when the current is high because of the distance (10 m plus). Again the readings are consistent between my battery app and my victron app.

I have frigged the settings short term to increase the absorption rate to 14.6v and duration to 4 hours just until I get the victron controller relocated.

My question is will this solve the problem? My solar panels (2x85 and 1x120) connected in parallel. Would they output more if they were in series (I am mostly in full sun or full shade so each panel gets roughly the same). I understand that series increases the voltage but reduces the amps and vica versa for parallel. What do I need amps or volts?

Many thanks for any help

battery charging
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16 Answers
wkirby avatar image
wkirby answered ·

All we know so far is that you have 290W of PV. That's good, but we could also do with a few more bits of information.
What is your nominal battery Voltage?
How much charge current did you observe during the day? Better still, how much charge power did you observe?

There is no such thing as current drop in a single circuit. The current will be equal in any part of a single circuit, but it will be lower if there is additional loss in the cables. If your wiring is adequately dimensioned, then I don't how relocating the controller would help. If the wiring is undersized then relocating the controller would help to reduce the Voltage drop.
Voltage drop can be measured from one end of the circuit to the other. The difference, as a function of the current, is determined by the resistance of the cables.

It was a hot day today, so the output of your panels will be reduced. For example, on a cool sunny day, my array makes more than 3.6kW peak. Today, despite the sunshine, the maximum was 2.7kW because the temperature of the panels was pretty high.

Do you need Amps or Volts? What you need is the product of the two, power (Watts).
The Voltage and current that you see from you panels should differ from the Voltage and current towards your battery. However, the power should be similar. There will be losses in the controller its self, but not very much.

When you say you fudged the settings to 14.6V, what were they before?
If your battery did indeed reach the target absorb Voltage and then held that before switching to float then it's full and low PV production would be a result of the power having nowhere else to go.
Were your loads satisfied throughout the day with the battery maintaining its float Voltage?

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helpless answered ·

Thanks for responding there’s a lot I don’t know the answer to but I will try and attach a few screenshot that may answer the questions you have asked on power and current.

As for voltage drop, apologies the one detail I failed to mention was that the dealer estimates the cable is 4 to 6 mm dia and the controller is about 10 to 12 meters from the batteries. The view was we are not getting full benefit in high generation conditions ( full sun either side of midday).

Today’s screenshot shows we saw 950wh which is the best I have seen. The day that started me questioning this was 8 days ago when we were out all day with everything switched off and SOC went from about 71% to 76 and we only saw 220wh but perhaps it was a duller day than I thought as I note pmax was only 70 that day. Was this a function of the light/cloud or would the battery BMS or solar controller affect this? I changed the settings on Monday this week (910wh generated and it didn’t switch to float)

Difficult to be precise about SOC gain on a daily basis as it depends on whether we were driving what we consumed etc but I would expect at this time of year this system to get us from 76% back to full if nothing is consumed.

Last couple of days readings not reliable as it was under cover

The default settings for lithium was 14.2 to absorption and 13.5 to float and 2 hours absorption time. I notice the charge has kept bulk and absorption going almost all day whereas a week or so ago it switched to float by lunchtime. One morning by 10am (I seem to recall).

In answer to the question did the set up fulfill our needs we have managed but I have only achieved 100% when on hook up. I have a small kettle that takes 5% from each battery when we use it and I would expect to be able to use three or four kettles in the morning and replenish during the day. Put another way. If I switched on my alde heating controller the current draw would have completely taken the full days charge. Here in June virtually the longest day of the year, what can I hope for in the winter when I need the heating on and we only have short days and poor sunlight?

My previous van only had 80 to 100w solar and it delivered far greater SOC recovery than this one is

Another question if hypothetically both absorption and float were set to 14.6v (consistent with battery supplier’s maximum) could I expect the battery BMS to protect against overcharge?

Sorry this is very long winded and thanks for giving this your time

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helpless answered ·

A404AC72-99F3-4D02-AA80-21CCFF87667D.png

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232f14f7-f36e-41d2-8d8d-ec4488464b46.png

Last week system went to absorption by about 9am and float by about 11. As you can see SOC had a long way to go

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46db3a33-d7a9-4b2f-9d0e-f0b2013cdc00.png

This reading was taken at the same time as the battery screenshot showing <50%

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4284088a-7031-49f1-ada7-add688615e85.png

Taken Sunday the same day as <50% shot

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9a02617f-08a9-4f52-aea1-f35fd8a975be.png

Settings prior to increasing 14.2 to 14.6 and 2 hours to 4 hours

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a2adc9f7-e92a-4ebb-8535-ef3d3018b288.png

Taken at same time as next shot Showing battery only charged to 49% by end of the day

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38d15bb9-24f7-4c93-8b61-b94ffb2ce0f0.png

The battery SOC after seeing 720wh with no significant drain during day but most of the day spent on absorption or float

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kevgermany avatar image
kevgermany answered ·

Your file didn't come through.

I was back in the UK over May/early June. Struggled to get 50% out of my panels, back in Germany I did a load test, got over 100%. Assuming your panels are flat on the roof, you'll lose a significant amount of the theoretical max power, quite apart from clouds etc.

The charge controller will give you values for voltage, power output and so on on your phone. These will help tell if there's a problem. The current out of the controller to the battery is of special interest. But if there are other loads at the time, these will take away from the output of the panels and reduce charge to the battery.

Would be worth checking and noting what other loads you have. Fridge for instance.

Not good to increase float voltage like that, BMS overcharge protection is a safety measure.

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helpless answered ·

@kevgermany & @wkirby Apologies I can’t get the photos to load keep getting an error. If anyone has the know how to get photos from my iPhone (I think they are too large) grateful for help?

Trying to address #kevgermany points . Nothing is switched on in the van no fridge no alde controller no inverter nothing. The only draw perhaps 1 amp is what the Moho controller takes.

Today I saw at lunchtime (not full sun) 150wh instantaneous which showed roughly 8.5A at the Solar reading and 10 at the battery ( using victron app) and the battery app confirmed roughly 4.9A on each. By then it was on absorption but with the adjustment I have made it stayed at absorption nearly all day. I was looking to see if the amperage got significantly higher than 10 but it never got fully sunny when I was available.

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Any further thoughts gents?

@kevgermany @WKirby

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helpless answered ·

ec7f4869-2433-40a2-8f03-4a2ebe09bf7c.png

Today’s screenshot


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helpless answered ·

5bff8edf-139c-4b03-b819-42d55ad1f009.png

Previous 4 days


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helpless answered ·

Today’s batteries SOC

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helpless answered ·

k

Last Sunday

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d576ac2f-83e1-4ea2-8301-95bb7ae4c3d1.png

This 650 wh only got us to 49% SOC

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helpless answered ·

1461f364-5863-4186-ac70-462071373c69.png

Soc after seeing 540wh during the day


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Sten avatar image
Sten answered ·

If you have 12m between the controller and the battery, and the current is 20A there Will be around 1.5V voltage drop in 6mm2 wire. I Think this is your problem. The controller think it has reach the absorption voltage but it has not.

As design always place the controller as close to the battery as posible.

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helpless answered ·

This was my n float despite SOC being less than 50%


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kevgermany answered ·

Please create a trend graph when it's charging. Use the - key to increase to time period, I think you can get as much as 2 hours. Ideally this will cover the time when it switches from absorption to float.

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Here is the trend for 10 minutes just after switching from bulk to absorption . SOC dropped 3% on each battery overnight and has now recovered but already switched from bulk. a9f27861-39f5-4f2b-97c5-4bc4e18a7825.pngImage Caption

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Revised settings and light levels preventing migration to float at the moment. We are back on bulk now as voltage dropped to 13.9
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helpless answered ·

be5d286b-170f-4dd9-835d-8c5f3a323ad2.pngTrend graph for 10 minutes shortly after switching from bulk to absorption with bar chart and battery readout showing SOC. I notice the battery voltage is quite different on the battery app than the victron app


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Sorry, missed your update.


The voltage here is the panel voltage at the mppt. Explains, maybe, the differences.

What battery voltage is the mppt seeing when charging? The long cab,le will affect this as pointed out by @Sten



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helpless answered ·

0e8a7858-bfa4-4d1a-9f36-adfd5e18aaee.pngSOC battery app


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Interesting that the battery only shows 13.5V when charging. Yet it's at about 80%,
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helpless avatar image helpless kevgermany ♦♦ ·
Yes I was on ehu Sunday night and the battery topped out at 14.3v when fully charged and on float for a few hours.

It doesn’t get much above 13.5 (the battery reading) when charged by solar although the victron readings are regularly 14.6v.

I do think moving the controller is going to have a big impact on the voltage seen and hopefully the charge gained

Any comment on whether panels are better in series?

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Won't help here, but not a bad move
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trevor-bird answered ·

If you are concerned at the location of your MPPT controller simply install a “smart battery sense” on the battery bank. This will provide via Bluetooth the actual battery terminal voltage to the mppt controller overcoming any voltage drop issues on the battery side of the controller.

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