Test Evpower cell board

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Re: Test Evpower cell board

Postby karrak » Tue Mar 15, 2016 3:27 pm

australsolarier wrote:my bcu also has a SOC counter. but it is highly inaccurate if you go longer than a day. 56V absorb on the 48V battery bank resets it to 100%. the battery in and out meter is more accurate and is much better to see than the small read out on the midnite classic. so when you go into the shed you see at a glance the batV and charge/discharge current. (midnite classic could/ should manufacture a large display as an accessory....)


I am fairly sure that Rod uses a magnetic hall effect sensor to measure the battery current. On a good day these are only accurate to around 2% and suffer from hysteresis.

You can get very good accuracy by using a simple current shunt and adjusting the current reading to take into account the battery 'current leakage'. In my case I get readings with an accuracy better than 1%SOC if the counter was reset on the previous day and better than 10%SOC if the counter was reset 20-30 days before the reading.

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Re: Test Evpower cell board

Postby australsolarier » Tue Mar 15, 2016 3:44 pm

yes,i think that is what it is called , magnetic hall effect, what is built into rod's blue ev power box. low currants it (1A) it is not very accurate. (the battery wire goes through ring)

to give you an example: 5 days no float, although the voltage rose over float voltage every day. the hall sensor says 84% SOC, the midnite shunt 95%. (lowest SOC 89%). to reset the ev power SOC, the voltage needs to go to 56V. the currant reading is acceptable but i wouldn't rely on the SOC unless it has been reset the day before.
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Re: Test Evpower cell board

Postby australsolarier » Tue Mar 15, 2016 3:49 pm

as for BMS, i remember vaguely electrodacus advertising his pwm solar charger her in this forum, and dozens of other fora for that matter, had a built in bms. if i remember vaguely he claimed 1mV and balancing on lower voltages. he said his charger could be used for bms only.
however there were a lot of things he did and was saying that i could not agree with or doubted very much.
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Re: Test Evpower cell board

Postby offgridQLD » Tue Mar 15, 2016 4:07 pm

Yes I pay no attention to the display on the EV power blue box it's useless.

Though I never intended to use it as a meter just the controller and contactor portion. That screen and meter was just a token add on that wasn't a option to order without it.

I do all my SOC and voltage readings from the Current shunt on my battery monitored by my classic charge controllers. As mentioned plotting out the logged data given it resets every day at float it's hallways within 1%. Actually I would say it's often less than 1% as it always ticks over to 100% seconds befor hitting float our would have seconds after when looking at the accumulative numbers.

Though really you only really need to be within 5% or so and its fine unless your taking your pack down to 10% SOC or something silly like that all the time.
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Re: Test Evpower cell board

Postby APR » Tue Mar 15, 2016 4:22 pm

For SOC I am using a Victron BMV-600 with a current shunt, and I believe it to be pretty accurate. I have not gone more than 2 days without achieving 100% SOC on the second day, and the Victron unit is displaying 100% SOC around 10 minutes prior to the charger shutting down current to the cells.

I also have the Victron BMV-600 actuating a Gigavac relay in a battery lead on under and over voltage of the pack to isolate the pack, however, an over or under volt event hasn't occurred yet in normal operation. As yet I don't have battery isolation occurring on single cell under or over voltage but I am looking at it. I want tighter cell voltage tolerances than are given by the EV Power balancers I have on the cells.
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Re: Test Evpower cell board

Postby australsolarier » Sat Mar 19, 2016 3:04 pm

spending a lot of time the last two days in the shed:

whilst bringing up the voltage via the midnite classic to 57.6v to balance the 48v lifepo4 battery the blue ev power box started to peep, reporting cell fault and then activated the disconnect relays. at least i know that works.
for some reason one of the cells must have gone over 4v. so i hooked the 12v kettle (about 1 ohm) on the cell for a while. (a few minutes) this solved the problem. (i suspect i charged that cell a bit too much whilst trying to balance yesterday)
the red leds on the balancing board whilst at first lighting up in different strenghts seem to get more equal after a while, though i interfered withg manual charging as well. (at 57.6v only about 1a went into the battery, so there would not have been much balancing anyway.)

as for the selectronic inverter after the disconnect it peeped too. the laptop in the house as well. though the main battery cable was disconnected, the selectronic has precharge wires. and this didn't turn off the selectronics 100%. after the battery was reconnected, the selectronic started up on his own again.
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Re: Test Evpower cell board

Postby Tracker » Mon Mar 21, 2016 7:08 am

offgridQLD wrote:........I also think in a Stationary storage (offgrid house) application you really need to balance your cells at a time when there is minimal load on the cells. Or at least a very stable load. ....


That could be where a RedFlow battery would be good as a (expensive) backup.. with their very long life potential, they could provide a reliable alternative power when.doing maintenance on the main battery..
I still believe that they would be a.good primary battery , despite their claimed 80% efficiency, which would just mean providing that much extra solar capacity.. (solar panels are relatively cheap)
..
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Re: Test Evpower cell board

Postby offgridQLD » Mon Mar 21, 2016 8:35 am

Tracker,
I do have 32 cells two x 16 cell 48v 40ah lithium banks that would happily run the house loads while balancing

Image

Though By the time you switched over to the other bank for just the inverter but not the charger (as you need the charging still on the house bank it for balancing) it would be just as easy to switch the AC breaker on the inverter for 30 min. If the wife and kids can't go without the TV for 30 min then I have bigger issues than a out of balance battery :lol:

Seriously though as long as the loads are reasonably steady (normal base loads on the home 300w or so +/- 100w or so . I don't have any issues keeping a stable low charge rate in high balancing absorb. As I always time the final balancing to when the cells have really tapered off in charge current.

It's more so for when your taking voltage readings to . You don't want to be reading cell one with 200w load on the bank and then cell 14,15, 16 with 1200w load from the microwave on the bank. As your voltage reading are not comparable. Though it depends on how your reading the cell voltage. If you have individual cell level metering then at a glance you can see all 16 cells voltage. With one MM its a 2min time frame from cell one to cell 16's readings you need stability.

One thing with instruments that give specifications on there tolerances. Don't always trust the specs. Particularly if its a very low cost import. I have 3 cell level monitors (Not cell log 8's) another brand and style.

The way I do a quick test is the comparison method. The same as you do with a spirit level . Check a surface in one direction then flip it around. If the readings are not the same in both directions it's junk.
.Connecting all three of them to one 8 cell bank the readings where all totally different on each cell and off by a long shot. Then comparing them to a accurate bench meter all three where not just off by a long shot to one another all three where off by a long shot to the true reading on each cell. Talk about leading you up the wrong path.

They have sat in a draw for years :lol: Though the listed specs would have you believe they are more than accurate enough for the task...just a shame they don't even come close to the specs.

Kurt
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Re: Test Evpower cell board

Postby APR » Wed Oct 05, 2016 11:04 pm

A couple of days ago I checked the voltage of my LiFePO4 cells and found one cell just under 3.9 volts. Charging had terminated in the preceding 5 to 10 minutes. The shunt across this cell was not passing current, and no leds were lit. It appears this shunt/balancer has for some reason failed. Looking further, another shunt has no leds lit. I pulled the high cell down with a 0.65 ohm resistance load.

A few weeks ago I took delivery of dynamic balancers from the Electric Car Parts Company, and have been going to relocate the battery pack before fitting these new balancers, however, I fitted them straiight away because of the failed EV Power balancer.

So far, with the new balancers, the cell balance at the top of charge has had a variability of around 30 millivolt, and after a kWh or two has been removed from the pack the cell voltages are very accurate as can be seen in the video of voltage checking the row of cells on the top shelf of my battery cabinet. The cells had been drawn down by a couple of kWh of house load, and the balancers had been fitted to the cells for around 8 hours.

https://youtu.be/TiHDirrmGcM
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Re: Test Evpower cell board

Postby australsolarier » Thu Oct 06, 2016 6:54 am

hi apr,
you really have to check the voltages just before float. although your measurements are impressing as far as equal cell voltages go, it is much more important before float switch over as far as top balancing is concerned.
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