Clamp meter

[VanKitten]

I just got a 400 amp AC/DC clamp meter.     

It has been driving me cray that I cannot get the battery meter wired into the system to tell me the remaining AMPs in that battery.    I am supposed to guess...because I never know for sure what value I started with.

So, anyone use a clamp meter?    I want to begin by testing the battery itself.    I think when I use this on the + battery cable..I am really getting a reading of the amps flowing there (in or out).     If I use it directly on the battery terminals...will this ensure I get the amp reading of the battery itself?

 

[Trebor English]

If you have one pair of golf cart batteries you can put the clamp on the wire that goes between the two batteries.  Usually the plus end has a bunch of connections and the minus has fewer.  Either end of the battery will work.  All the current has to go through the clamp to measure all the current.  If you are only looking at the inverter then only an inverter wire gets clamped.  

The clamp on amp meter only shows how fast current is flowing.  It doesn't keep track of in and out or how much is left.  A hydrometer will tell you the battery state of charge unless you have a sealed battery.

 

[VanKitten]

These are lithium. Total 12v, 400 amp

They will almost always test between 13.3 and 13.1. Volts. Volt meter is worthless with LiFePo4

 

[Trebor English]

For the purpose of tracking a battery state of charge a clamp on amp meter is not sufficiently stable. A "Hall effect transducer" is the same thing without the moving parts. You have to thread the wire through the hole. A shunt resistor can also work. You will still be cray.

 

[John61CT]

RK, from the wording of your OP I suspect you may misunderstand something fundamental here.

An ammeter, whether clamp type or shunt based, measures amps, not amp hours.

Amps is an instantaneous measure of flow rate, like gallons per minute. Just like watts, which are independent of voltage.

So a clamp ammeter will show 60W light bulb at 12V is burning 5A while it is on full power. With a dimmer switch turning it down, the ammeter will show it using 4A, 3A, 2A etc.

Now, amp hours (AH) is a static quantity of energy. If that lamp is using 5A, when it has burned for one hour it has used 5AH.

Say a 100AH LFP battery has usable 90AH. That means it will feed that lamp for 18 hours. If, you turn the dimmer down to a usage rate of 2A, then you can run the lamp for 45 hours.

I hope this above helps you understand that this sentence

> It has been driving me crazy that I cannot get the battery meter wired into the system to tell me the remaining AMPs in that battery.

does not make sense. Nor does this:

> If I use it directly on the battery terminals...will this ensure I get the amp reading of the battery itself?

A battery just sitting there, whether full or empty, does not store any amps. If you have a load drawing power from the battery, an ammeter will show at what **rate** the batt is getting discharged.

What you want to find out is SoC, State of Charge, how many AH are left. An ammeter does not do that.

In fact AH remaining cannot be measured directly at all, but only estimated by a proper battery monitor. More accurately by some than others, of course this usually means more expensive.

Since SmartGauge does not work with LFP, that leaves only shunt-based "coulomb counter" type battery monitors.

Only ones that require the user to input data on the battery, like

AH capacity
Peukert factor
Charging (Coulombic) efficiency

and have the ability to "reset" the meter at the 100% Full point, best based on tailing endAmps

stand a chance of calculating (estimating) SoC with any accuracy.

The user must reset the Full point manually every few cycles or they drift, accuracy declines.

As AH capacity declines, the user must give a BM updated accurate value.

My preferred BM is the Victron 702-BMV.

Hope this helps.

 

[John61CT]

Additional reading

http://www.pbase.com/mainecruising/battery_monitor

http://www.pbase.com/mainecruising/programming_a_battery_monitor

____
For people with lead banks, SmartGauge is more accurate and **much** more user friendly, does not require programming those battery coefficients nor AH capacity, and automatically adapts as that capacity walks down over time.

http://www.pbase.com/mainecruising/smart_gauge

 

[VanKitten]

Problem is this..... how do I know when it is full so that I can reset the monitor at that time?

I always read 13.1. Always. Which is meaningless

I was hoping that someone would tell me that it would read 14.6 volts or so when full...but that does not seem to be the case

So...all I really know is that at 13.1 the battery is somewhere between 80-90% full and 80% discharged....which is to say...no knowledge at all

So...I cannot know the state of the charge...ever. Just guessing without knowledge.

Must be a better way

 

[John61CT]

> how do I know when it is full so that I can reset the monitor at that time?

My definition of Full for longevity is:

Set charge source at 13.8V (3.45pc). Voltage with no source active will be lower.

Start charge source and measure V at terminals. Observe V climbing as bank accepts current.

Stop when bank V = 13.8.

____

If I had a lot of loads to immediately start feeding and wanted to stuff as many AH as possible into the bank at that moment:

Holding at 13.8V, use the ammeter to watch amps decline.

Stop when Amps falls to .02C (2A per 100AH bank capacity).

____

Now if you don't agree that Absorb = 13.8V and endAmps = .02C

then you can set those variables however you like.

Whatever your setpoints may be, the process is the same

 

[John61CT]

> I always read 13.1. Always. Which is meaningless

Sounds like you don't have an accurate voltmeter. Where are you reading that, with what tool? *

______
> I was hoping that someone would tell me that it would read 14.6 volts or so when full

Remember the voltage of the battery at rest - no load, no charge for 24+ hours - has nothing to do with the voltage of the circuit while charging is active.

I've never seen an LFP batt show anywhere near even 14V resting.

And would never allow anything that high during routine operations.


_____
> So...all I really know is that at 13.1 the battery is somewhere between 80-90% full and 80% discharged....which is to say...no knowledge at all

> So...I cannot know the state of the charge...ever. Just guessing without knowledge.

> Must be a better way

Yes, voltage while bank is in use does not reflect SoC accurately. Call it Level 1

My posts above give you the information you need to get as accurate as possible. Call that Level 5

Accuracy between those, say Level 2 is possible with a cheap shunt based AH counter -not what I would call a proper Battery Monitor - resetting at Full manually each cycle, and watching the AHs counting down.

With an **accurate** voltmeter, you will probably learn to get pretty close correlating between the two, because there is no way resting Volts are actually flat all the way from SoC 100% Full (however you define it) down to where your BMS cuts out (call that 0%).

* Or, lots less likely, if that is a drop-in battery, maybe the internal BMS circuitry is regulating output?

Please state the battery make and model.

 

[John61CT]

Here are a couple trustworthy examples of charging and discharging graphs.

Using CALB CA180FI cells, a quality product and pretty standard for mobile House use, as opposed to relatively obscure and untested drop-in models.

Four in series to get to a nominal 12V block, so take the per-cell voltages and multiply times four.

Note the charge and discharge rates (amps) are higher than most people here use, so these curves are a bit steeper than we'd usually see.

Note where the curves start to more steeply change at the top and bottom.

These are the "shoulders" to be avoided if going for maximum longevity, in my case 3.05V at the bottom and 3.45V at the top.

Doing so you sacrifice a few % of usable AH capacity, but likely triple or quadruple the rated lifecycles from mfg ratings.

Which is IMO why they want users to follow *their* recommendations for defining "Full".

Another note is that you really do want accurate measuring tools, two decimal places on voltage is a minimum.

And even professional level meters costing hundreds of dollars need to be periodically calibrated.

But the voltmeter included in the Victron BMV when purchased new should be "good enough"

 

[VanKitten]

Thank you John for the graphs. That helps me understand.

Just one comment I will add here. The supplier I purchased these batteries from is in Salt Lake..and is totally useless. There were made in China..and this guy knows nothing about the product he sells....and is either not willing to contact the manufacturer or they don't bother to talk to him. Either way, I get none of my questions answered. Avoid this guy!!
Electriccarparts

 

[John61CT]

Same outfit as discussed here?

http://www.diyelectriccar.com/forums/showthread.php?t=105314

If so I suspect they're used to selling to EV hobbyists that already know their stuff, and spend thousands at a shot.

Not excusing, them, but. . .

More to the point which battery model?

 

[jonyjoe303]

On your lithium battery pack can you read each group of cells battery voltage? that is what what will tell you if your battery is fully charge. 

On a 12 volt lifepo4 battery pack (4s batterypack) each group of cells should read 3.65 volts, when all 4 groups read 3.65 volts (total 14.6 volts) you got a full battery thats also in perfect balance. 

On my 110ah lifepo4 I only see that when I charge it with a balance charger. And it takes a while up to 5 hours if it is out of balance. Once the charger starts balancing it reduces output to less than an amp. Without the balancing and slow charging it, I can probably get it up to 105 ah, which is good enough.

This is the coulomb meter I been using, you might need something like this (cost 22 dollars) its simple but has been doing a great job on my lifepo4. As long as you can track how many amps you use the day before, you know how much need to go in the next day. Can be use on up to a 600 ah battery bank.

[font='Helvetica neue', Helvetica, Verdana, sans-serif]TK15 High Precision LiFePO/Lithium[/font][font='Helvetica neue', Helvetica, Verdana, sans-serif]/Lead Acid Battery Tester Coulomb Counter 50A[/font]

 

[John61CT]

Again, you are drastically shortening that expensive bank's lifetime by going that high.

Stay below 14 and there also are no balance issues, yes check on them every six months or so, but many users cycling more gently report no balancing needed at all for many years.

And I believe Kat's is a 12V sealed up drop-in, no access to the cells nor the internal BMS.

 

[tripper]

My LiFePo4 is a 12V drop in. 
I am running A Victron Solar controller, which has 13.XX digits of resolution, it is very accurate (0.03 off)so I do not need an external voltmeter ( I am a 22 USAF Metrologist, compared Victron to a calibrated Fluke DMM).

I have Bulk/Absorb on it set to 13.95V.
I have float set to 13.1

This arrangement seems to be working very well. The battery seems to sit around 13.13 volts, it takes about 20 minutes of use ( 4 amp load or so) before the float even kicks in, meaning when it is in float mode it is not even trickle charging, just letting it sit at 0 current until a load has been applied for a while, then the float kicks in and matches the current use.

The voltmeter built in on the Victron is accurate enough to get a decent idea of the batteries SOC. This is at night, as my nighttime loads are small so the battery voltage is a good representation of what I have remaining. During the day my solar is enough that I don't pull energy from the battery.

The Victron controller charges up to Bulk voltage for about 1 1/2 to 3 hours in the morning, usually reaching bulk voltage (13.95 in my case) by 10:00 am, then kicks into absorb mode, which is very short, trickling down to almost no current within 20 minutes or less, then goes into float mode (13.1) where it cuts off all power to battery as the battery voltage of 13.13 is higher than the float.  Float will power all my power hungry accessories when I kick them on, it will only go back into Bulk mode if I exceed my solar power input. 

I  occasionally look at the voltage if I am using a lot of power over a long time (like charging 300 watt e bike battery and 300 watt gaming laptop at same time), otherwise I totally just ignore the system, it just runs reliably with zero user input needed from me. I just glance at the voltmeter on the 12V fridge to see if the system is within normal range during the day.

To me, voltage seems to be a good way to measure the available power in the lithium, it does not seem to sag under use like the lead acid battery. The Victron set this way has worked well for me, I guess time will tell.

 

[John61CT]

Relevant thread that Maine Sail just contributed to, commenting on many "murderous" mistakes he sees DIYers make, especially in the RV/van markets

http://www.cruisersforum.com/forums/showthread.php?p=2522639

 

[John61CT]

> My LiFePo4 is a 12V drop in. 

which one?

 

[John61CT]

> Bulk/Absorb on it set to 13.95V

I would suggest a little lower, at 13.80V (3.45Vpc)

would gain years of lifetime cycles with very little sacrifice of AH capacity at the top end, in your use case not a sacrifice at all since you're not cycling deeply.

Also, make sure you keep at low SoC levels when there is no or very low load cycling, it is quite harmful to longevity for an LFP bank to sit near Full for even short periods of time.

I highly recommend your reading my queries to MaineSail in the link I provided and give your comments or take the (rare) opportunity to ask him an questions you may have.

I would also appreciate your feedback on my trying-to-be-comprehensive but concise summary I posted there.

 

[tripper]

> My LiFePo4 is a 12V drop in. 

which one?

Its a random Ebay one. About $650 with built in BMS. Has been working great so far.

 

[tripper]

> Bulk/Absorb on it set to 13.95V

I would suggest a little lower, at 13.80V (3.45Vpc)

would gain years of lifetime cycles with very little sacrifice of AH capacity at the top end, in your use case not a sacrifice at all since you're not cycling deeply.

Also, make sure you keep at low SoC levels when there is no or very low load cycling, it is quite harmful to longevity for an LFP bank to sit near Full for even short periods of time.

I highly recommend your reading my queries to MaineSail in the link I provided and give your comments or take the (rare) opportunity to ask him an questions you may have.

I would also appreciate your feedback on my trying-to-be-comprehensive but concise summary I posted there.

Thanks, I will drop the bulk down to 13.8, I was testing it and did notice a small loss in total capacity (voltage) when charged to this lesser state. Actually i think I will plug the Whynter fridge into the lead acid Trojan (has a separate 180 watt panel) and drop the lithium cell to half SOC and turn off the solar to it for now, I'm satisfied that it is working well now, I've been testing it for several months.

I will check out your summary on MaineSail, thanks