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Discussion Starter · #1 ·
I've learned a lot about batteries and electrical systems. I do have a question regarding battery disconnects? Is there such a product that can disconnect a battery automatically at a specific voltage or SOC? I'm looking at LIFEPO4 lithium battery and want to cut off the battery at 80%, but I'm not sure how I could go about doing that? It would be for both 12v loads and to the inverter. Sorry if this is a newbie question.
 

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I'm a complete noob but the charger-inverter will often have the
ability to 'look' at the battery bank and shut down at a voltage you
select/determine preventing dropping the voltage below a certain point
and prolonging the life of your batteries---but I thought a much less
concern with lithiums.
Also some appliances also have this ability. The ARB frigerator if you wish
can also look for battery depletion and cut off saving you from having to
walk.
The pros here will chime in soon enough.
I was just googling at what voltage I'd need to preven below 50% discharge
of my AGMs.
 

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I'm using an InPower LVD20. Here's the info:

http://www.inpowerdirect.com/lowvoltagedisconnects.php
http://www.inpowerdirect.com/docs/PDS-124A.pdf

It is totally self contained and very simple. Loads that you want to disconnect under low voltage conditions are feed through the LVD20. If the voltage drops below 11.5 volts for longer than 60 seconds the load is disconnected. To reconnect the voltage must be greater than 11.5 volts and the control pin is cycled off and then on.

The folks at InPower are also very helpful if you contact them.

Lex
 

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Is there such a product that can disconnect a battery automatically at a specific voltage or SOC? I'm looking at LIFEPO4 lithium battery and want to cut off the battery at 80%
Matt,

What lithiums are you looking at? Most lithiums, even the so-called lead-acid replacement/drop-ins, include a Battery Management System (BMS) to protect against over and undervoltage. Ours has a large solenoid, controlled by the BMS, that 'dumps" all loads if any cell voltage drops below 2.8 volts (for 30 seconds).
 

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Discussion Starter · #5 ·
Not sure yet. I was looking at Stark Power, but their max discharge current is only 100amps and I want to use an induction cooktop so I don't think that will work. Plus their bms says the disconnect happens at "~10.0V". Is that like dead dead voltage? I guess you truly get what you pay for when it comes to these LIFEPO4 batteries.

What battery are you using @Winston?
 

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their bms says the disconnect happens at "~10.0V". Is that like dead dead voltage?
I don't think voltage is a very reliable method of determining SOC with LiFePo4 batteries because they will maintain 12-13 plus volts for a much longer time during discharge compared to AGMs.

This is a benefit with things like inverters, diesel furnaces, diesel cooktops and refrigerators that won't work properly without high enough voltage but it's problem for devices with low voltage cutouts and automatic battery combiners that depend on typical AGM voltages to work properly.

I think you need a shunt based battery monitor that calculates SOC from battery capacity and amp hours used to accurately determine the 80% discharge level you want to stop at. Trimetric makes one and Victron makes one with a programmable relay control that might be capable off triggering a relay when a certain SOC is reached.

There is an induction cooktop that only uses 750 watts or 60 - 70 amps at 12v. And all induction cooktops use less than maximum watts at lower settings even if only by turning off and on to maintain the desired heat setting. Use a kill a watt meter to measure the amp draw of you cooktop during a typical cooking session to determine what your actual draw would be.

If necessary you could connect 2 100 AH Stark batteries in parallel then the amp draw would be 1/2 as much on each battery. Or maybe a larger battery would allow a larger amp draw.

I would call Stark Power and ask them if their battery couldn't handle a slightly higher discharge rate for the short time you would likely need with an induction cooktop.

I have Stark batteries and they have performed fine so far in the first year and the people at Stark seem very knowledgeable and helpful.

What I like most is the light weight and how fast they charge from the alternator.
 

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. . . but their max discharge current is only 100amps and I want to use an induction cooktop so I don't think that will work. Plus their bms says the disconnect happens at "~10.0V". Is that like dead dead voltage?[/MENTION]?
Matt,

By coincidence we were playing with our induction stove-top today and have been investigating 'discharge' issues with both Elite and Starlight Solar (the people responsible for our batteries and BMS).

We had run the SOC down to 0%, yet the cell voltages were running 3.26 - 3.27 volts. This seemed way too high for the alleged zero percent State of (dis)Charge. In fact, we'd been running the system for days at 0% - - mostly low power stuff and the occasional power tool. But just having re-wired our AC distribution, we fired-up the TruInduction cooktop to 'watch water boil" - - how exciting! :~) We ran it for about 20 minutes (remember, this is three days after our batteries reported a SOC of 0%). First comment, the current never exceeded 100 amps . . . the highest we saw was 85 amps.

Now, jumping to what we've learned about 'voltages' (and it is true, as tgregg noted above, that voltage is less than a precise way of measuring SOC, except, maybe, at the extremes of near zero and near 100% charge), we were told to never let a cell drop below 2.5 volts - - which is equivalent to the 10 volt level that Stark indicated was their cut-out.

But, good BMS's do not rely on 'pack voltage', i.e. that 10 volts you mentioned, rather, each cell is monitored. When you hit the extremes, cell voltages begin to diverge so, if you don't shut down discharging until the 'entire pack' reaches 10 volts, it is certain that one or more cells is below 2.5 volts.

Our BMS monitors each cell and dumps all loads if any cell drops below 2.8 volts.

The sense we got was that 3.2 volts approximates a 20% SOC, 3.1 volts is 10% and that at 3.0 volts, the cell is essentially fully discharged, that further discharge results in a rapid decline of cell voltage. Incidentally, our BMS gives us a low voltage/discharged warning when the pack voltage hits 12.0 volts. This happened today at the end of our water boiling fun.

So, in summary, we suspect you can run an induction cook-top from a Stark 100AH battery. We don't like their protection. But if you include a good SOC monitor and don't fall asleep (while boiling water), you should be ok.

Winston
 

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Discussion Starter · #8 ·
If necessary you could connect 2 100 AH Stark batteries in parallel then the amp draw would be 1/2 as much on each battery. Or maybe a larger battery would allow a larger amp draw.

I would call Stark Power and ask them if their battery couldn't handle a slightly higher discharge rate for the short time you would likely need with an induction cooktop.

I have Stark batteries and they have performed fine so far in the first year and the people at Stark seem very knowledgeable and helpful.

What I like most is the light weight and how fast they charge from the alternator.
I called Stark this afternoon and the person I spoke with seemed to have idea what I was talking about in terms of discharge rate. She kept going back to capacity. "It has 1600wH of capacity". So two batteries in Parallel, I'll always be pulling half the amp draw from each battery if there are two present?

Matt,

By coincidence we were playing with our induction stove-top today and have been investigating 'discharge' issues with both Elite and Starlight Solar (the people responsible for our batteries and BMS).

We had run the SOC down to 0%, yet the cell voltages were running 3.26 - 3.27 volts. This seemed way too high for the alleged zero percent State of (dis)Charge. In fact, we'd been running the system for days at 0% - - mostly low power stuff and the occasional power tool. But just having re-wired our AC distribution, we fired-up the TruInduction cooktop to 'watch water boil" - - how exciting! :~) We ran it for about 20 minutes (remember, this is three days after our batteries reported a SOC of 0%). First comment, the current never exceeded 100 amps . . . the highest we saw was 85 amps.

Now, jumping to what we've learned about 'voltages' (and it is true, as tgregg noted above, that voltage is less than a precise way of measuring SOC, except, maybe, at the extremes of near zero and near 100% charge), we were told to never let a cell drop below 2.5 volts - - which is equivalent to the 10 volt level that Stark indicated was their cut-out.

But, good BMS's do not rely on 'pack voltage', i.e. that 10 volts you mentioned, rather, each cell is monitored. When you hit the extremes, cell voltages begin to diverge so, if you don't shut down discharging until the 'entire pack' reaches 10 volts, it is certain that one or more cells is below 2.5 volts.

Our BMS monitors each cell and dumps all loads if any cell drops below 2.8 volts.

The sense we got was that 3.2 volts approximates a 20% SOC, 3.1 volts is 10% and that at 3.0 volts, the cell is essentially fully discharged, that further discharge results in a rapid decline of cell voltage. Incidentally, our BMS gives us a low voltage/discharged warning when the pack voltage hits 12.0 volts. This happened today at the end of our water boiling fun.

So, in summary, we suspect you can run an induction cook-top from a Stark 100AH battery. We don't like their protection. But if you include a good SOC monitor and don't fall asleep (while boiling water), you should be ok.

Winston
Thanks Winston.
 

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I called Stark this afternoon and the person I spoke with seemed to have idea what I was talking about in terms of discharge rate. She kept going back to capacity. "It has 1600wH of capacity". So two batteries in Parallel, I'll always be pulling half the amp draw from each battery if there are two present?
Sounds like you spoke to Elysa Higgs, office manager who was very helpful with delivery dates, billing, etc. Try asking for Martin Koebler. His business card says CTO. Telephone 704 804 4464 or 800 587 9940. Email [email protected]

The amp draw from each of 2 parallel batteries will be equal assuming the wire lengths and wire sizes between the load and the 2 batteries are the same. This means the negative wire of the load is connected to one battery (through the shunt if you have a battery monitor) and the positive wire of the load is connected to the other battery and the wires connecting the 2 negative terminals and the 2 positive terminals are the same length and gauge.

Otherwise there would be slightly more draw from the battery with the shortest electrical path to the load.
 
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