I came across this and thought I'd share it. Its a inexpense way to monitor battery charge, similar to Link 10, except not as many features. Bob F.
Battery Discharge Gauge
- Thread starter Bob F.
- Start date
- Status
Not a Charge Monitor
The ENM “PT27" is nothing at all like a link 10 Battery Monitor. What it is, is a digital clock, and a very inaccurate voltmeter. Voltage (alone) is a nearly useless indication of state-of-charge. It’s not worth the posted $70 price.
The ENM “PT27" is nothing at all like a link 10 Battery Monitor. What it is, is a digital clock, and a very inaccurate voltmeter. Voltage (alone) is a nearly useless indication of state-of-charge. It’s not worth the posted $70 price.
Battery Expert
I worked extensively with a NiCd Battery expert, probably one of the top ten. He said that there are only two times you can tell the state of charge of a battery. That is when it is fully charged and when it is fully discharged. Everything else is at best an educated (or uneducated) guess.
I worked extensively with a NiCd Battery expert, probably one of the top ten. He said that there are only two times you can tell the state of charge of a battery. That is when it is fully charged and when it is fully discharged. Everything else is at best an educated (or uneducated) guess.
State of Charge
Current state-of-charge technology has extended the "state-of-art" much beyond the realm of educated guesswork. There are several (expensive) strategies for accurately measuring (&/or calculating) state of charge. Your (NiCad) expert is NOT wrong - but he IS overstating his argument.
Current state-of-charge technology has extended the "state-of-art" much beyond the realm of educated guesswork. There are several (expensive) strategies for accurately measuring (&/or calculating) state of charge. Your (NiCad) expert is NOT wrong - but he IS overstating his argument.
Gord-I disagree
There are too many variables. The equipment we used was incredibly sophisticated, but we still got in trouble occasionally. The batteries I am talking about are what are referred to as the NASA standard NICD. They were generally 50AH or 20AH. They could cost up to a million dollars for a three battery set. Other similar problems exist for other chemistries. The variables which really screw everything up is the age of the battery which includes the cycles the battery has completed, the cycle depths, the temperature, the temperature of the environment and probably most important and the most unquanatifiable is the specific chemistry of the specific battery being used. In spite of mass production, there are differences between batteries. Also not an issue for the batteries we are talking about a NICD has special problems. There is the memory effect which means that you have a 50AH battery, but if it is only used to discharge 15AH repeatedly, it will suddenly run out of useable capacity at around 15AH,
There are too many variables. The equipment we used was incredibly sophisticated, but we still got in trouble occasionally. The batteries I am talking about are what are referred to as the NASA standard NICD. They were generally 50AH or 20AH. They could cost up to a million dollars for a three battery set. Other similar problems exist for other chemistries. The variables which really screw everything up is the age of the battery which includes the cycles the battery has completed, the cycle depths, the temperature, the temperature of the environment and probably most important and the most unquanatifiable is the specific chemistry of the specific battery being used. In spite of mass production, there are differences between batteries. Also not an issue for the batteries we are talking about a NICD has special problems. There is the memory effect which means that you have a 50AH battery, but if it is only used to discharge 15AH repeatedly, it will suddenly run out of useable capacity at around 15AH,
Monitors
It really doesn't matter much, because I'm not convinced that anybody who feels that an amp-meter (LINK 10, etc.) is necessary really has any clue as to what's happening with their batteries. That's because people who need gages can't do the math and energy budgets. I have a non-functioning fuel gage (6 YEARS) and have NEVER gotten close to running out of fuel. Guys WITH FUEL GAGES are spending too much time with Vessel Assist. 1/2 gallons per hour, how many hours = # gallons use, it's simple match. Same is true for battery banks. Energy budget says: fridge 30 AH per day Lights 10 autopilot 5 etc. Total load, say, 60 ah per day. Battery bank: 400 ah 80% of 400 is 320 available, to 50% 200, difference is 120. 60 ah per day to 120 ah available is 2 {freaking'} days. Who needs a gage? Do the math, please. Stu PS The $$$ you spend on a gage is better spent on another battery to enlarge your house bank. Why measure a small bank, when you can last longer and go further with a bigger bank? A Link 10 costs what, $100 by the time it's installed, even if you do your own work. So how about 2 Costco batteries to bring your feeble stock house bank from 150 amp hours up to 400. What a deal!!! PPS Before anyone starts questioning my fridge amp hours, we turn it off at night. It's dark it's cold, we're in Northern California, and we get away with it. Maybe not in Maryland, but that's OUR budget. As they say in the small print, yours may differ. Golly, do the math, YOURS.
It really doesn't matter much, because I'm not convinced that anybody who feels that an amp-meter (LINK 10, etc.) is necessary really has any clue as to what's happening with their batteries. That's because people who need gages can't do the math and energy budgets. I have a non-functioning fuel gage (6 YEARS) and have NEVER gotten close to running out of fuel. Guys WITH FUEL GAGES are spending too much time with Vessel Assist. 1/2 gallons per hour, how many hours = # gallons use, it's simple match. Same is true for battery banks. Energy budget says: fridge 30 AH per day Lights 10 autopilot 5 etc. Total load, say, 60 ah per day. Battery bank: 400 ah 80% of 400 is 320 available, to 50% 200, difference is 120. 60 ah per day to 120 ah available is 2 {freaking'} days. Who needs a gage? Do the math, please. Stu PS The $$$ you spend on a gage is better spent on another battery to enlarge your house bank. Why measure a small bank, when you can last longer and go further with a bigger bank? A Link 10 costs what, $100 by the time it's installed, even if you do your own work. So how about 2 Costco batteries to bring your feeble stock house bank from 150 amp hours up to 400. What a deal!!! PPS Before anyone starts questioning my fridge amp hours, we turn it off at night. It's dark it's cold, we're in Northern California, and we get away with it. Maybe not in Maryland, but that's OUR budget. As they say in the small print, yours may differ. Golly, do the math, YOURS.
State-of-Charge (SOC)
Of course, I should have said that current state of the art Battery Monitors are RELATIVELY accurate. In our application on boats, the accuracy probably exceeds our ability to use the information. I think STU put it pretty well into perspective, though - spend about $500 on a good battery monitor; or about half that on a couple of hundred amp/hours of additional battery capacity? I realize that NASA doesn’t have the luxury of adding the weight of additional batteries, and that they would need very accurate SOC information. I also realize that they spent millions of dollars developing a pen that would write in Zero-G - the Russians used a pencil. Notwithstanding, Voltage (OCV) alone is a pretty poor indicator of state-of-charge. An approximate voltage, such as the $70 device that opened this topic, is useless.
Of course, I should have said that current state of the art Battery Monitors are RELATIVELY accurate. In our application on boats, the accuracy probably exceeds our ability to use the information. I think STU put it pretty well into perspective, though - spend about $500 on a good battery monitor; or about half that on a couple of hundred amp/hours of additional battery capacity? I realize that NASA doesn’t have the luxury of adding the weight of additional batteries, and that they would need very accurate SOC information. I also realize that they spent millions of dollars developing a pen that would write in Zero-G - the Russians used a pencil. Notwithstanding, Voltage (OCV) alone is a pretty poor indicator of state-of-charge. An approximate voltage, such as the $70 device that opened this topic, is useless.
SOC
I can agree with the "relatively accurate" point. Certainly you have to keep track of the amp hours somehow, but no method is completely accurate. The older the batteries and the higher the discharge rates or any thing else out of the ordinary, the more inaccurate the SOC becomes. It is easy to compute the amount of AH removed. What you don't know is the amount of AH remaining and even possibly how close the battery was to a full SOC to start with. The problem with using voltage for SOC is that it can not be done accurately real time, i.e. I am out in the lake and need to know whether my trolling motor is about to run out of power. If there is a load on the battery or it is being charged, then there is no OCV to use. The OCV can be estimated using battery impedance (if you happen to know it), but that is not very accurate. If you take the load off the battery or stop charging, then you must wait for an hour or so to let the OCV stabilize. The pencil is an interesting point. I have never worked for NASA, but have worked with them. We were not allowed to use pencils for anything that had to do with an official document for obvious reasons. Pencil entries can be changed. I have seen ink entries reviewed with a magnifying glass to see if the "official" entry had been changed after a problem. The other problem with pencils is the contamination they can cause. We were not allowed to take them into a clean room. I don't know whether this applies once they get out of the clean room and into space, but I would bet that they don't like pencils in space for the same reason.
I can agree with the "relatively accurate" point. Certainly you have to keep track of the amp hours somehow, but no method is completely accurate. The older the batteries and the higher the discharge rates or any thing else out of the ordinary, the more inaccurate the SOC becomes. It is easy to compute the amount of AH removed. What you don't know is the amount of AH remaining and even possibly how close the battery was to a full SOC to start with. The problem with using voltage for SOC is that it can not be done accurately real time, i.e. I am out in the lake and need to know whether my trolling motor is about to run out of power. If there is a load on the battery or it is being charged, then there is no OCV to use. The OCV can be estimated using battery impedance (if you happen to know it), but that is not very accurate. If you take the load off the battery or stop charging, then you must wait for an hour or so to let the OCV stabilize. The pencil is an interesting point. I have never worked for NASA, but have worked with them. We were not allowed to use pencils for anything that had to do with an official document for obvious reasons. Pencil entries can be changed. I have seen ink entries reviewed with a magnifying glass to see if the "official" entry had been changed after a problem. The other problem with pencils is the contamination they can cause. We were not allowed to take them into a clean room. I don't know whether this applies once they get out of the clean room and into space, but I would bet that they don't like pencils in space for the same reason.
- Status