Before heading south from Ct. last month, decided we needed some form of refigeration on board. So, bought an office size frig, an inverter and two 105AH deep cycle batteries. Older house battery was a single and fairly old. The inverter is a 2000 Watt, inverter/charger. All installed and working as it should far as I can tell. Not the problem. Can not keep the house bank up with the frig on for an entire night. Batteries are just too far gone to run the fridge all night. When the batteries are charged up, either from the alternator, or the charger, they show the proper voltage. Between 12 1/2 and 13 volts. This is on the on board voltmeter, and with a digital. A hydrometer shows one battery to have one two bad cells, and the other to be about half charged. I have disconnected and tried each seperately. Still the exact same readings. Either one by itself will run things for approx. three hours before it is too low to kick the inverter on. The fridge, according to the info plate, draws 0.95 amps at 110V. By my way of eyeballing numbers, it should draw approx 10 amps at 12 volts. With a 50% run rate, this thing should still run all night on a single battery. I have the exact same setup on my boat, and it will easily run everything for 24 to 36 hours and still have enough left in the batteries to crank the engine. The only thing I am sure of is that something is wrong. Any ideas out there. I know there are some really sharp people on this board, so any answers, ideas or suggestions would be greatly appreciated. Thanks Phil Prater
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Batteries
Ampere hour ratings are generally based on a twenty hour discharge rate. so 105 AH batteries should be able to supply about five amps each for twenty hours. But by your admission one of the batteries is failing and the draw by the fridge is at the limit of the other. What are your other loads? Are you living in the dark. If you are still in the chasapeake then most of your food will keep in a bucket on deck especially at night. We have been making ice on deck some nights. Phil, Another question. You stated in your post that you have the same set-up on your boat. So whose boat are you on? Did you purchase new batteries or are these batteries of some unknow age and ancestory? OK two questions.
Ampere hour ratings are generally based on a twenty hour discharge rate. so 105 AH batteries should be able to supply about five amps each for twenty hours. But by your admission one of the batteries is failing and the draw by the fridge is at the limit of the other. What are your other loads? Are you living in the dark. If you are still in the chasapeake then most of your food will keep in a bucket on deck especially at night. We have been making ice on deck some nights. Phil, Another question. You stated in your post that you have the same set-up on your boat. So whose boat are you on? Did you purchase new batteries or are these batteries of some unknow age and ancestory? OK two questions.
Yes
get a 12 v DC marine system. A 120 AC home system is not efficient enough for 12 DC and an inverter.
get a 12 v DC marine system. A 120 AC home system is not efficient enough for 12 DC and an inverter.
Reserve minutes
I'm no electrical expert, but I agree with your math. With a 10% increase in amps for the inverter, you are still running only 10 amps. If both batteries are hooked together and the refrig is the only load, then you should be able to run about 10.5 hours to 50%, recharge and go again. Three hours on each battery seems to me to either indicate one of three possibilities: 1. The inverter is extremely inefficient 2. The refrigerator is malfunctioning and your actual load is more like 30+ amps. 3. If your actual load is in excess of 25 amps, you are going to have considereably less amp hours available due to also having to consider reserve minutes as well as amp hours when chosing a battery. Have you checked the actual 110 volt load on the refrigerator? Is it already cooled and at a steady state or is it trying to overcome the heat to cool food just loaded? What is the model of your inverter?
I'm no electrical expert, but I agree with your math. With a 10% increase in amps for the inverter, you are still running only 10 amps. If both batteries are hooked together and the refrig is the only load, then you should be able to run about 10.5 hours to 50%, recharge and go again. Three hours on each battery seems to me to either indicate one of three possibilities: 1. The inverter is extremely inefficient 2. The refrigerator is malfunctioning and your actual load is more like 30+ amps. 3. If your actual load is in excess of 25 amps, you are going to have considereably less amp hours available due to also having to consider reserve minutes as well as amp hours when chosing a battery. Have you checked the actual 110 volt load on the refrigerator? Is it already cooled and at a steady state or is it trying to overcome the heat to cool food just loaded? What is the model of your inverter?
The hydrometer wont lie.
It seems to me your batteries may be sulfated and are not accepting full charge and thus discharging quicker. Brand new batteries sitting on the shelf at the store for more than a year can become sulfated. If your hydrometer is a quality instrument I would trust its readings. It is telling you that one battery only accepts 1/2 charge and that the other has two bad cells. This is quite possible even in brand name batteries.
It seems to me your batteries may be sulfated and are not accepting full charge and thus discharging quicker. Brand new batteries sitting on the shelf at the store for more than a year can become sulfated. If your hydrometer is a quality instrument I would trust its readings. It is telling you that one battery only accepts 1/2 charge and that the other has two bad cells. This is quite possible even in brand name batteries.
Amp hour rating
Most deep cycle battery manufacturers define amp-hour capacity as the amp-hour discharge required to arrive at a 1.75 volts/cell (10.5 volts/12 volt battery) reading at 80 degrees F over a 20 hour period. The salient point here is that you should not expect to use those 105 amps you noted as battery capacity because many 12 volt devices (your inverter/fridge combo for example) will cease to work LONG before you reach a 10.5 volt reading. Further, discharging your battery to this level is not healthy for it and isn't recommended. As a rough rule of thumb you can actually expect to use approximately 50% of your batteries stated capacity, or 52.5 amps to still remain above a useable voltage of at least 11.8 - 12 volts which is generally considered to be a safely but completely discharged battery. All of this assumes a healthy battery in top notch condition and a steady discharge without big spikes. If you have large discharges over a short period the actual capacity of your battery is reduced further. If, as Jerry points out, you're starting your weekend with a warm fridge that causes the unit to run frequently for the first few hours then you won't even see the 52.5 hours mentioned earlier and may have a 'functionally' discharged battery after, say, 6 or 8 hours. I can't speak to your specific set up but I can say that as a rule the 'dorm' style fridges are very inefficient as evidenced by your 10 amp observation. I'll bet with your inverter losses you're using more than that. We have a 'dorm' style Norcold unit that is standard on Hunter Legend 40's and it is a real hog so we shut it down when away from the dock for any length. Our Grunnert 12v unit in our ice box is much more efficient at 4-5 amps with much less run time. A good set up for you might be one of the self-contained 12 volt frig/freeze 'coolers.' You could load it at home, cool it at home on an AC converter and then bring it to the boat ready to go. Hope this helps.
Most deep cycle battery manufacturers define amp-hour capacity as the amp-hour discharge required to arrive at a 1.75 volts/cell (10.5 volts/12 volt battery) reading at 80 degrees F over a 20 hour period. The salient point here is that you should not expect to use those 105 amps you noted as battery capacity because many 12 volt devices (your inverter/fridge combo for example) will cease to work LONG before you reach a 10.5 volt reading. Further, discharging your battery to this level is not healthy for it and isn't recommended. As a rough rule of thumb you can actually expect to use approximately 50% of your batteries stated capacity, or 52.5 amps to still remain above a useable voltage of at least 11.8 - 12 volts which is generally considered to be a safely but completely discharged battery. All of this assumes a healthy battery in top notch condition and a steady discharge without big spikes. If you have large discharges over a short period the actual capacity of your battery is reduced further. If, as Jerry points out, you're starting your weekend with a warm fridge that causes the unit to run frequently for the first few hours then you won't even see the 52.5 hours mentioned earlier and may have a 'functionally' discharged battery after, say, 6 or 8 hours. I can't speak to your specific set up but I can say that as a rule the 'dorm' style fridges are very inefficient as evidenced by your 10 amp observation. I'll bet with your inverter losses you're using more than that. We have a 'dorm' style Norcold unit that is standard on Hunter Legend 40's and it is a real hog so we shut it down when away from the dock for any length. Our Grunnert 12v unit in our ice box is much more efficient at 4-5 amps with much less run time. A good set up for you might be one of the self-contained 12 volt frig/freeze 'coolers.' You could load it at home, cool it at home on an AC converter and then bring it to the boat ready to go. Hope this helps.
How low is too low to run the inverter?
How do you know the batteries are 'fully charged'? That generally takes more time than an hour or two on the alternator, which will get them to perhaps 80%. The last 20% can take a lot more time. If you just bought the batteries, I suggest you take them back to the dealer and have him test them - that will put that question to bed very quickly. Otherwise, take them to any auto parts store and ask them to test them. I second Waffle's recommendation... is that a first??? I also agree with Benny and Landsend about the hydrometer... but if one battery shows 2 bad cells, it shouldn't even be able to run the inverter. Hydrometers don't lie, but their data needs to be bench-marked to have any real meaning. You mentioned that you have the same setup on 'your' boat and it runs for 24 - 36 hours.... I'm confused... As far as usable capacity, most cruisers only count on 30%, from 80% to 50% of capacity. Here's the logic: As I mentioned before, it takes HOURS to get from 80 to 100%... not efficient to run an engine just to put those amps in. Consider that the recommended charging rate (Capacity / 8) only puts 13 amps back into the battery per charging hour) If you consistently drain your batteries below 50% you severly affect their life. I suspect that you have a very inefficient system with 2 suspect batteries. There are many good books on this subject - I recommend Nigel Calder's 'Boatowner's Electrical and Mechanical Handbook' - lots of good stuff in there. Cheers, Bob s/v X SAIL R 8
How do you know the batteries are 'fully charged'? That generally takes more time than an hour or two on the alternator, which will get them to perhaps 80%. The last 20% can take a lot more time. If you just bought the batteries, I suggest you take them back to the dealer and have him test them - that will put that question to bed very quickly. Otherwise, take them to any auto parts store and ask them to test them. I second Waffle's recommendation... is that a first??? I also agree with Benny and Landsend about the hydrometer... but if one battery shows 2 bad cells, it shouldn't even be able to run the inverter. Hydrometers don't lie, but their data needs to be bench-marked to have any real meaning. You mentioned that you have the same setup on 'your' boat and it runs for 24 - 36 hours.... I'm confused... As far as usable capacity, most cruisers only count on 30%, from 80% to 50% of capacity. Here's the logic: As I mentioned before, it takes HOURS to get from 80 to 100%... not efficient to run an engine just to put those amps in. Consider that the recommended charging rate (Capacity / 8) only puts 13 amps back into the battery per charging hour) If you consistently drain your batteries below 50% you severly affect their life. I suspect that you have a very inefficient system with 2 suspect batteries. There are many good books on this subject - I recommend Nigel Calder's 'Boatowner's Electrical and Mechanical Handbook' - lots of good stuff in there. Cheers, Bob s/v X SAIL R 8
Bad Battery, Bad..
If you have tested the batteries and have found one with bad cells, and they are in parallel, that could be your culprit. A bad battery in parallel with a good battery will influence the charge and capacity of the other. A flooded battery usually has six cells at approx. 2.25 to 2.3 volts per cell, for a total of 13.25 to 13.8 volts. If you have two bad cells the voltage can show it as being charged, called gas voltage, but when under load it quickly dies. If this setup is working for you, I would suspect both batteries are bad. Most any place that sells batteries can load test them. Your logic is correct as for the Amp conversion from DC to AC,10 to 1, but unless you check it with a ampmeter, it is a guess. An inverter can lose quite a lot of power from heat loss, much like an gasoline engine. This is the efficiency rating of an inverter. Sail On... Terry
If you have tested the batteries and have found one with bad cells, and they are in parallel, that could be your culprit. A bad battery in parallel with a good battery will influence the charge and capacity of the other. A flooded battery usually has six cells at approx. 2.25 to 2.3 volts per cell, for a total of 13.25 to 13.8 volts. If you have two bad cells the voltage can show it as being charged, called gas voltage, but when under load it quickly dies. If this setup is working for you, I would suspect both batteries are bad. Most any place that sells batteries can load test them. Your logic is correct as for the Amp conversion from DC to AC,10 to 1, but unless you check it with a ampmeter, it is a guess. An inverter can lose quite a lot of power from heat loss, much like an gasoline engine. This is the efficiency rating of an inverter. Sail On... Terry
Bad Battery in Series
If you have a bad battery in the series, it will draw the others down. Even with a bad cell, the battery will read 12+ volts, but the capacity is still reduced. For a trip such as yours, I would have started out with new batteries. You can always add one at-a-time to the series as you need it.
If you have a bad battery in the series, it will draw the others down. Even with a bad cell, the battery will read 12+ volts, but the capacity is still reduced. For a trip such as yours, I would have started out with new batteries. You can always add one at-a-time to the series as you need it.
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