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Old 27-01-2013, 05:00   #1561
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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It must be relative to full scale. What else could it be? The numerical accuracy can hardly go up as the voltage reading drops.
Sorry, "full scale" is a strictly analog concept. The accuracy of a DVM is usually specified as 0.X% of the reading, plus N "counts". IIRC the count figure is the fundamental VDC accuracy for the range. In the DVM world, total accuracy actually goes down the further the reading is from zero, as each additional "bit" introduces its own tiny error, which accumulates.
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Old 27-01-2013, 05:04   #1562
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Heck Doug, I remember rectifier tubes before diodes. Needed that DC B+ voltage for our triode tube before we had transistors. Quite a transition back then, going from voltage gain devices to current gain devices.
Bob, you must be really old! I remember those crazy tube tester contraptions in drug stores from my early childhood---but only vaguely

Don't feel too bad though. I am convinced that others on this thread still use whale blubber oil for their boat lighting.
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Old 27-01-2013, 05:45   #1563
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Just a warning that it is easy to delude yourself that your measurements are accurate when measuring millivolts in 4 volts. The meter's digits look so confident.
That's certainly true. Until you pointed it out, I always thought my 4VDC scale was accurate to the millivolt. I'll only report voltages to two places for now on.
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Old 27-01-2013, 05:47   #1564
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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An interesting piece from Capt Mike motor sailing the Hudson River going to NYC. In the beginning before he had any wind or current assist, it looks like he was able to do 3.2 kt on about 720 watts from the battery. I couldn't help but calculate his range if he bought those 700 a-hr cells that Balqon still has on their clearance sale. At 3.2 kts with zero currents or wind assist, Capt Mike could motor for 160 nm on (16) 700 a-hr cells in series for his 48 volt propulsion motor.
THE BIANKA LOG BLOG: ELECTRO SAILING AROUND NEW YORK: PART ONE
Deckofficer:

Interesting. What kind of depth of discharge would that be for LiFePO4 bank?
In the 40 nm motor sail I made I operated under AGM battery alone for the first two and half hours. My current modus operandi with my EP system is that when the battery bank capacity drops to around 75 to 80% from 100% on the battery meter then I go into hybrid mode with the Honda generator. I keep current draw at around 15 to 16 amps because that is all the Zivan 48 volt NG-1 charger can provide in hybrid mode. As shown it pushes my 8 ton 30 foot sailboat along at around 3 knots. Yeah, I do baby the 10kw AGM 48 volt battery bank and charge early and often. But, the bank is still going strong as I begin my sixth season in a few months. Of course solar and a wind turbine keep things topped up at the mooring too. Since I'm setup for the AGM bank and it has worked well for the past five years I don't feel any urgent need to change it out yet. It's simplicity of monitoring, charging and cost of each battery still has me giving it the edge for now. Though I'm keeping an eye on the LiFePO technology too and see if they hold up as well as in the long term.
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Old 27-01-2013, 08:22   #1565
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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OK to use a cell logger to signal say a 200amp breaker to shut the discharge off in the event of 'over or under' voltage on one or more cells, what actual components do you need in a circuit to activate a heavy breaker like that from a small signal?

Diagram?

Cheers
Try something like this. If you feel opto-isolation is required just throw it in, between VCC(+12V) and +ON.

Parts are not critical. You can find very nice N-FETs on old PC motherboards (next to CPU) and opto-isolator is in most all PC power supplies.
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Old 27-01-2013, 09:09   #1566
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Quote:
Originally Posted by deckofficer
Heck Doug, I remember rectifier tubes before diodes. Needed that DC B+ voltage for our triode tube before we had transistors. Quite a transition back then, going from voltage gain devices to current gain devices.
To keep you happy we invented FETS

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Old 27-01-2013, 09:28   #1567
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For actual live monitoring, I am using an Arduino-like microcontroller with a 12-bit ADC, scaling up to a max of 17 volts. 12 bits is 2^12 or 4096, so it is also accurate to 17/4096 = 4mV. Thus I only display cell voltages to the hundredth.
Each "step" is 4mV. Once calibrated to a multimeter that has some degree of measurement error. Then surely there is some tolerance in the ADC itself? Then some drift in the voltage dividing resistors on temperature? I don't know the answer, but I would like to understand the precision possible...
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Old 27-01-2013, 09:40   #1568
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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To keep you happy we invented FETS

Dave
I thought it was Dr. Oskar Heil. I was a lab tech at ESS (the speaker) and we bought from Oskar the patent on his air motion transformer to be used as a high audio frequency transducer on our speakers. Totally cool guy.
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Old 27-01-2013, 09:48   #1569
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Deckofficer:

Interesting. What kind of depth of discharge would that be for LiFePO4 bank?
In the 40 nm motor sail I made I operated under AGM battery alone for the first two and half hours. My current modus operandi with my EP system is that when the battery bank capacity drops to around 75 to 80% from 100% on the battery meter then I go into hybrid mode with the Honda generator. I keep current draw at around 15 to 16 amps because that is all the Zivan 48 volt NG-1 charger can provide in hybrid mode. As shown it pushes my 8 ton 30 foot sailboat along at around 3 knots. Yeah, I do baby the 10kw AGM 48 volt battery bank and charge early and often. But, the bank is still going strong as I begin my sixth season in a few months. Of course solar and a wind turbine keep things topped up at the mooring too. Since I'm setup for the AGM bank and it has worked well for the past five years I don't feel any urgent need to change it out yet. It's simplicity of monitoring, charging and cost of each battery still has me giving it the edge for now. Though I'm keeping an eye on the LiFePO technology too and see if they hold up as well as in the long term.
Close to 100% DOD, but even at 80%, that would be a lot of nm per charge. You probably don't have room for (16) of those 700 a-hr cells. The way you treat your AGMs, I can see why you get such good cycle/storage life from them.
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Old 27-01-2013, 09:52   #1570
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Bob, you must be really old! I remember those crazy tube tester contraptions in drug stores from my early childhood---but only vaguely

Don't feel too bad though. I am convinced that others on this thread still use whale blubber oil for their boat lighting.
Not that old, just was into electronics as a young kid. My first HAM transmitter was a Knight-Kit T-60 AM CW in 1963. I did build a Knight-Kit tube tester. Heath Kits were too expensive for me.
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Old 27-01-2013, 10:17   #1571
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by deckofficer View Post
Heck Doug, I remember rectifier tubes before diodes. Needed that DC B+ voltage for our triode tube before we had transistors. Quite a transition back then, going from voltage gain devices to current gain devices.

A whole bunch of TV/Radio repairmen retired or quit the business over that very transition.
The same thing happened again when integrated circuits came out, replacing transistors.

In those days I had to read trade magazines every night to keep up with the new technology, and I had really thick skin on my fingers from changing hot tubes like type 80, 5U4, 6DQ6 etc.
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Old 27-01-2013, 10:40   #1572
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Asked our supplier who strongly recommends a BMS for the installation if the BMS cards themselves should be fused and if so what size to prevent any fire from card failure.
Cheers
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EVERY wire & control boards added must be made safe & 100% prevented from adding to fire. i.e. adding circuit board type fuses to all circuit boards & wires used for the add on circuits.
Regards Bill
I agree with the above and would use instrument fuses to protect the BMS unless the Printed Circuit Boards include some adequate over current protection . Casual wiring as seen at 1400, 1542 is just an accident waiting to happen.

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Dr. Frankenstein-like technology,
It's Frankensteen":
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Old 27-01-2013, 10:50   #1573
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I agree with the above and would use instrument fuses to protect the BMS unless the Printed Circuit Boards include some adequate over current protection . Casual wiring as seen at 1400, 1542 is just an accident waiting to happen.

In defense of post 1400, I used Maine Sail's test set up in my post. I can assure you that Maine Sail does excellent marine system's work and what the picture depicted was just a series of cell testing procedures and not a finished product for the sail boat.

Sorry Maine Sail for using your picture and having someone misunderstand because I didn't include your text. I won't do that again.
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Old 27-01-2013, 11:16   #1574
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Casual wiring as seen at 1400, 1542 is just an accident waiting to happen.
1542 is mine. Some points:

1/ It's a test setup not an installation.
2/ Look closely, all the monitor wires are fused at the battery terminals.
3/ I know the load test wiring is not safe. READ THE POST: "My test rig is not quite up to unattended operation, so I did not start a load test"
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Old 27-01-2013, 12:40   #1575
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

For what I can see from 1542 is that a red wire from the positive of the first cell is connected next to a black from the negative of what appear to be the fourth cell at what may look like the plug of a data logger. I may be missing it but I can see no apparent over current protection in that wiring. Anyway good that you mention that all the wires are over current protected.

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There is also a tremendous amount of energy potential in a fully charged bank that deserves a lot of respect, just like any other battery. But even more so since it will deliver it almost all at once.
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