After having read the Ample Power Primer, Nigel Calder's book etc. I have a question. Everything I have read indicates that 20% to 25% of battery capaicty is a good rule of thumb when sizing the charging system, alternator or shorepower to the capacity of a battery bank Does this percentage refer to the total capacity of the battery ie. 90Ah or the amount past which it is unwise to discharge, 50% or 90 Ah or 45Ah? Suppose I have a 12v deep cycle which has a stated capacity of 90Ah. If the charging system output in amps should equal 20%-25% of battery capacity, is output calulated on total capacity, 90Ah thus 18A - 22.5A output? Or is it 50% of the total 90 Ah capacity, which in this case is 45Ah thus 9A - 11.25A output? If it is the total capacity then won't I end up with a system that is essentially twice as large as necessary? Totally amped out! David
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How long does it take to charge???
that is the basic problem. A 1 Amp charger will eventually get most systems fully charged. Of course it may take days!!!! But you hear people swear by the little 300MA solar chargers!!!! Is it your only charging system??? As I motor into the dock after a weekend of sailing my 45 Amp alternator is hard at work plus incidental motoring during the weekend and light loads so my batteries shouldn't be that low at the end of the weekend. So basically I need to top off the batteries. Since I won't be back for at least a week a 1 amp charger is OK. I have an installed 20 amp that rarely pulls more than 5 amps. The short answer is that unless you want to fully charge a fully discharged system in 5 hours why use 20-25%??? Tom
that is the basic problem. A 1 Amp charger will eventually get most systems fully charged. Of course it may take days!!!! But you hear people swear by the little 300MA solar chargers!!!! Is it your only charging system??? As I motor into the dock after a weekend of sailing my 45 Amp alternator is hard at work plus incidental motoring during the weekend and light loads so my batteries shouldn't be that low at the end of the weekend. So basically I need to top off the batteries. Since I won't be back for at least a week a 1 amp charger is OK. I have an installed 20 amp that rarely pulls more than 5 amps. The short answer is that unless you want to fully charge a fully discharged system in 5 hours why use 20-25%??? Tom
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David W
How large is your battery bank(s)?
What shorepower charger do you have installed? How have you monitored your remaining capacity when you state "my batteries shouldn't be that low at the end of the weekend." Just curious as use a digital voltmeter to check mine. Once they drop to around 12.10v I get concerned. I will have 200 AH of house capacity (total capacity, not the 50% dicharged portion) and a starting battery. Simple math would say that if I never let batteries go past 50% discharge I have to replace 100 AH at most. At 10A rate that equals 10 hrs of charging, 20A equals 5 hours. That presumes that when charging I have absolutely no other drains on the system. I have no AC lights on board so when I am cruising and have access to shore power the DC system is still responsible for my lights etc. The charger then has to replace the discharged amps plus whatever is being used while at the dock. Thanks! David
What shorepower charger do you have installed? How have you monitored your remaining capacity when you state "my batteries shouldn't be that low at the end of the weekend." Just curious as use a digital voltmeter to check mine. Once they drop to around 12.10v I get concerned. I will have 200 AH of house capacity (total capacity, not the 50% dicharged portion) and a starting battery. Simple math would say that if I never let batteries go past 50% discharge I have to replace 100 AH at most. At 10A rate that equals 10 hrs of charging, 20A equals 5 hours. That presumes that when charging I have absolutely no other drains on the system. I have no AC lights on board so when I am cruising and have access to shore power the DC system is still responsible for my lights etc. The charger then has to replace the discharged amps plus whatever is being used while at the dock. Thanks! David
Charge rates
The “conventional” wisdom: Yes - 20-25% of RATED A/H capacity as follows (90Ah Rated x .20 or .25 = 18A or 22.5A output in your example) 1. A good rule-of-thumb is not to use a charger (or charging setting) for batteries that is greater than 25% of the AH capacity* (C/4) or 10% of the RC rating of the battery or batteries being charged (except 33% for AGM). * Based on the C/20 discharge capacity 2. The BULK stage is where the charger current is constant and the battery voltage increases, which is normally during the first 80% of the recharge. You can give the battery whatever current it will accept as long as it does not exceed 25% of the 20 hour (expressed "C/20") ampere hour (Ah) or 10% of the RC rating and wet batteries do get over 125 deg. F (51.5 deg. C) and VRLA batteries do not get over 100 deg. F (37.8 deg. C). 3. The typical "Trickle" Charger (1-5 amp) will NEVER recharge a battery higher than about 80-85% rated capacity. For an interesting & convincing alternative viewpoint, wherein the charging current can be much higher, I highly recommend the following reading: How "fast" you can reliably charge your battery. http://cruisersforum.com/showthread.php?s=&threadid=1432 & Understanding a model for battery charge acceptance. http://cruisersforum.com/showthread.php?s=&threadid=1442 HTH, Gord May
The “conventional” wisdom: Yes - 20-25% of RATED A/H capacity as follows (90Ah Rated x .20 or .25 = 18A or 22.5A output in your example) 1. A good rule-of-thumb is not to use a charger (or charging setting) for batteries that is greater than 25% of the AH capacity* (C/4) or 10% of the RC rating of the battery or batteries being charged (except 33% for AGM). * Based on the C/20 discharge capacity 2. The BULK stage is where the charger current is constant and the battery voltage increases, which is normally during the first 80% of the recharge. You can give the battery whatever current it will accept as long as it does not exceed 25% of the 20 hour (expressed "C/20") ampere hour (Ah) or 10% of the RC rating and wet batteries do get over 125 deg. F (51.5 deg. C) and VRLA batteries do not get over 100 deg. F (37.8 deg. C). 3. The typical "Trickle" Charger (1-5 amp) will NEVER recharge a battery higher than about 80-85% rated capacity. For an interesting & convincing alternative viewpoint, wherein the charging current can be much higher, I highly recommend the following reading: How "fast" you can reliably charge your battery. http://cruisersforum.com/showthread.php?s=&threadid=1432 & Understanding a model for battery charge acceptance. http://cruisersforum.com/showthread.php?s=&threadid=1442 HTH, Gord May
Gord are you saying that solar panels don't work??
From what I've read most solar panels put out less than 5 amps under ideal conditions and that is just a few hours a day. Many people on this board brag about their little 0.35 amp trickle chargers keeping their batteries charged while on a mooring etc. In my personal experience a 1.5 amp trickle charger will fully charge my 230AH house bank. It is self regulating and goes to floating after the batteries are fully charged. I have a 20 amp installed system that maxes out about 10 amps when the bank is really low as when I lost my alternator. But usually pulls less than 5 amps and then drops to just above zero. I wouldn't use a 1.5 amp as an only charger but it works!!!! Ask all of these people with small solar panels. 1.5 X 24 = 36 AH/day and few solar panels deliver that many AH's. A 100 watt panel is 100/15.5=6.5 amps under ideal conditions and that is only a few hours a day between about 10 am and 2 pm. A 100 watt panel is a fairly large panel. I am looking at replacing my old installed unit and a 10 amp is probably what I'll get. that is 5 amps per lead for a two bank system. So even if my batteries are pretty low overnight at a marina should fully recharge assuming 12 hours * 5 amps = 60 AH,s. As you have repeatedly noted a battery won't take but so many amps regardless of what the charger can provide. If you have a 5 amp charger getting a 10 amp won't cut time in half and definately a 20 amp charger won't cut time in fourth. It won't hurt to have a 20 amp or even a 100 amp charger as their regulators will reduce the current to what the battery will accept. But a regulated 100 Amp charger won't charge a 200AH system much faster than a 5 amp charger. Don Casey goes into the limits of using a large alternator. I forget the exact numbers but I think that going from a 45 amp alternator to a 100 amp alternator only saved about 15% in recharging time. After a few minutes at full output the 100 amp alternator starts cutting back until after a short time it is only putting out about the same as the smaller alternator. Shortly thereafter neither alternator is ptting out at full power. Tom
From what I've read most solar panels put out less than 5 amps under ideal conditions and that is just a few hours a day. Many people on this board brag about their little 0.35 amp trickle chargers keeping their batteries charged while on a mooring etc. In my personal experience a 1.5 amp trickle charger will fully charge my 230AH house bank. It is self regulating and goes to floating after the batteries are fully charged. I have a 20 amp installed system that maxes out about 10 amps when the bank is really low as when I lost my alternator. But usually pulls less than 5 amps and then drops to just above zero. I wouldn't use a 1.5 amp as an only charger but it works!!!! Ask all of these people with small solar panels. 1.5 X 24 = 36 AH/day and few solar panels deliver that many AH's. A 100 watt panel is 100/15.5=6.5 amps under ideal conditions and that is only a few hours a day between about 10 am and 2 pm. A 100 watt panel is a fairly large panel. I am looking at replacing my old installed unit and a 10 amp is probably what I'll get. that is 5 amps per lead for a two bank system. So even if my batteries are pretty low overnight at a marina should fully recharge assuming 12 hours * 5 amps = 60 AH,s. As you have repeatedly noted a battery won't take but so many amps regardless of what the charger can provide. If you have a 5 amp charger getting a 10 amp won't cut time in half and definately a 20 amp charger won't cut time in fourth. It won't hurt to have a 20 amp or even a 100 amp charger as their regulators will reduce the current to what the battery will accept. But a regulated 100 Amp charger won't charge a 200AH system much faster than a 5 amp charger. Don Casey goes into the limits of using a large alternator. I forget the exact numbers but I think that going from a 45 amp alternator to a 100 amp alternator only saved about 15% in recharging time. After a few minutes at full output the 100 amp alternator starts cutting back until after a short time it is only putting out about the same as the smaller alternator. Shortly thereafter neither alternator is ptting out at full power. Tom
sort of ...
Nope, I’m not saying that Solar Panels don’t work. It gets a little complicated, and I’m on overload right now. I suggest you read my earlier references (posted by Rick @ CruisersForum). I’ll try to expand my answer overe the weekend. I knew I shouldn’t offer a short opinion!!! Gord
Nope, I’m not saying that Solar Panels don’t work. It gets a little complicated, and I’m on overload right now. I suggest you read my earlier references (posted by Rick @ CruisersForum). I’ll try to expand my answer overe the weekend. I knew I shouldn’t offer a short opinion!!! Gord
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