Seems to be lots of confusion about batteries, charging them, how long they should last, etc. so I thought I?d put together some basic information for anyone wanting it.
Let?s start with the difference between a standard automotive cranking battery and a deep cycle battery used for trolling or a ?house? battery. A cranking battery needs to be able to produce very high amperage for a short amount of time. (Long enough to crank the engine over) A house battery needs to produce a lower amperage but over a longer span of time. Due to the very different needs the two types of batteries are constructed differently. A true deep cycle battery has solid lead plates which helps prevent them from warping when discharged heavily over time. An automotive battery has perforated plates and if you cut the battery apart the plates actually look a lot like a sponge. The greater surface area of the sponge like plates can produce higher amperages. (More acid in contact with more of the lead plates.) Why not just make them all bigger and with solid plates? Weight and size. A true deep cycle battery that was capable of providing the same amperage draw (high amps / short duration) as a cranking battery would be two or three times the size and weight of the cranking battery.
Now for the surprising part, most ?deep cycle? batteries sold for Marine / RV use are NOT true deep cycle in the sense they do NOT have solid lead plates and instead are a hybrid of the two types. In general, that is a good thing because in the same way a deep cycle battery cannot produce the high amps of an automotive battery, they can NOT be charged as fast an automotive battery without damaging them. Most boats are set up so that the engines alternator recharges the house batteries when the engine is running. The average engine alternator can produce 60 to 100 amps and that is FAR too high of a current for a true deep cycle battery so a hybrid version is used. If you want to use a true deep cycle battery (or bank of them) you must use a current limiting device to prevent over charging from the engines alternator. (Or replace the batteries far too often.)
Next up, determining the charge of the battery. Many people don?t understand when their battery is discharged. For all practical purposes a battery that is discharged to 10.5 volts is considered ?dead?. WARNING, discharging a battery beyond this point will ruin it in no time and it will never be able to be fully recharged again. It will take what is known as a ?surface charge? but you will never get the longevity from it again. In figure 1 are the approximate voltage / charge states. As an example, a battery that is 50% discharged should still read 12.06 volts with no load on it. A battery that is 80% discharged will have a voltage of 11.58 with no load. So how low should you discharge a battery? That really depends on how often you want to replace it. The further you discharge the battery, the more you cut down on its number of charge / discharge cycles. In figure 2 you can see the effect that deeply draining the battery has on its life. A battery that is routinely discharged to its maximum (10.5 volts) will provide less than 200 charge / discharge cycles while the same battery that is discharged by only 30% will provide over 1200 cycles. (Yes, it really is that big of a deal.) So what is the sweet spot? Most manufactures will say around 50%. That is based on what is practical. If you wanted to never go below 30% discharge you would need two batteries to produce the same amount of power and as batteries are big and heavy you don?t want to fill your boat with batteries.
At some point we will all have to test a battery to determine its condition and the best way to do it is with a hydrometer. Unfortunately a LOT of batteries sold as Marine / RV deep cycle batteries are sealed making testing with a hydrometer all but impossible. One of the big reasons I don?t buy sealed batteries, that and the fact I like to make certain they have the proper amount of electrolyte in them. With a hydrometer not only can you check the batteries over all condition, you can check each battery cell. To use a hydrometer you will want to fully charge the battery, let it sit 10 to 12 hours and then test. One of the key things you are looking for is that all the cells read the same or within .05 variance. If they aren?t all relatively close in specific gravity the battery is on its way to failure and should be replaced in the near future. In Figure 3 you can see what the charge state is by measuring the specific gravity of the electrolyte.
Charging rates are also very often misunderstood. Faster is NOT better! The typical deep cycle battery should be charged over 8 to 10 hours. Charging them faster shortens their life just as surely as discharging them too fast or too deeply will. So what size charger should be used? As noted above, manufactures recommend not going below the 50% discharge level so we will assume that is as far as we push them. Now let?s say it?s a 100 AH (Amp Hour) battery so we divide the AH rating by two (50% discharge) and then divide that by the time it takes to charge them, say 8 hours.
100 divided by 2 = 50 divided by 8 = 6.25. In this scenario the charger should NOT produce more than 6.25 amps at its maximum output.
If you have a bank of batteries, (say four) you add their AH together and do the same math. (Four 100 AH batteries)
4 x 100 = 400 divided by 2 = 200 divided by 8 = 25. In this scenario the charger should NOT produce more than 25 amps at its maximum output.
Yes I know, someone will chime in and say they charge them faster. Those people also replace them more often than they need to. ;-) Charging at a higher rate than is needed only wastes electrical power, overheats the batteries, and shortens the batteries life span.
There is an exception, this formula is for ?flooded? batteries. Gel batteries MUST be charged considerably slower and if you use them you should get the manufactures specifications on charge rates. In a flooded battery (standard lead acid) if bubbles are formed during charging they float to the top of the acid and pose no problem. In a Gel battery the bubbles can?t float and you end up with voids in the gel and that reduces the contact with the plates and you end up with a poorly performing battery.
Discharge rates can also be confusing due to how the AH rating is computed. There are two variables when considering AH ratings, the amps you can draw and what period of time you can draw them. The vast majority of ratings for AH are based on 20 hours of use / discharge. What does that mean in the real world? Lets again take a 100 AH battery as our average battery. In THERORY that battery should produce 100 amps for 1 hour. In the real world it comes nowhere close to that. At 100 amps draw that battery would be lucky to last 20 minutes. Again, the industry standard is to calculate AH based on 20 hours of discharge. That means you could pull 5 amps for 20 hours. Keep in mind that is with a new battery in perfect condition, as they age and go through the charge / discharge cycle that AH rating falls off. Most manufactures will provide a discharge rate (curve) based on load and time and it?s worth reviewing before buying! In my opinion there are two companies that stand head and shoulders above the rest when it comes to deep cycle batteries providing the most power for the longest period of time, Trojan and Crown. Unfortunately they charge a price commensurate with their product.
Maintenance chargers are something I?d like to see kicked to the curb. They work well enough on NEW batteries that are in top condition but reduce the life span as the batteries age. How so? Go back to figure 1 and you see that a fully charged battery should have 12.7 volts while a battery with only 90% charge has 12.5 volts. That is only a difference of .2 volts. Now let?s say your batteries are a year old and have been through 50 charge / discharge cycles. It may well be that they can only get to say 90% or 95% of the full charge they could obtain when new. The problem is, a maintenance charger has no way of determining if your battery is old or new and it will continue to try to charge it to 100%. As it can?t, it just sits there cooking the battery. In my opinion you are MUCH better off having a 2 amp trickle charger you put on the battery for a 3 or 4 hours once a month during non-use periods.
Let?s start with the difference between a standard automotive cranking battery and a deep cycle battery used for trolling or a ?house? battery. A cranking battery needs to be able to produce very high amperage for a short amount of time. (Long enough to crank the engine over) A house battery needs to produce a lower amperage but over a longer span of time. Due to the very different needs the two types of batteries are constructed differently. A true deep cycle battery has solid lead plates which helps prevent them from warping when discharged heavily over time. An automotive battery has perforated plates and if you cut the battery apart the plates actually look a lot like a sponge. The greater surface area of the sponge like plates can produce higher amperages. (More acid in contact with more of the lead plates.) Why not just make them all bigger and with solid plates? Weight and size. A true deep cycle battery that was capable of providing the same amperage draw (high amps / short duration) as a cranking battery would be two or three times the size and weight of the cranking battery.
Now for the surprising part, most ?deep cycle? batteries sold for Marine / RV use are NOT true deep cycle in the sense they do NOT have solid lead plates and instead are a hybrid of the two types. In general, that is a good thing because in the same way a deep cycle battery cannot produce the high amps of an automotive battery, they can NOT be charged as fast an automotive battery without damaging them. Most boats are set up so that the engines alternator recharges the house batteries when the engine is running. The average engine alternator can produce 60 to 100 amps and that is FAR too high of a current for a true deep cycle battery so a hybrid version is used. If you want to use a true deep cycle battery (or bank of them) you must use a current limiting device to prevent over charging from the engines alternator. (Or replace the batteries far too often.)
Next up, determining the charge of the battery. Many people don?t understand when their battery is discharged. For all practical purposes a battery that is discharged to 10.5 volts is considered ?dead?. WARNING, discharging a battery beyond this point will ruin it in no time and it will never be able to be fully recharged again. It will take what is known as a ?surface charge? but you will never get the longevity from it again. In figure 1 are the approximate voltage / charge states. As an example, a battery that is 50% discharged should still read 12.06 volts with no load on it. A battery that is 80% discharged will have a voltage of 11.58 with no load. So how low should you discharge a battery? That really depends on how often you want to replace it. The further you discharge the battery, the more you cut down on its number of charge / discharge cycles. In figure 2 you can see the effect that deeply draining the battery has on its life. A battery that is routinely discharged to its maximum (10.5 volts) will provide less than 200 charge / discharge cycles while the same battery that is discharged by only 30% will provide over 1200 cycles. (Yes, it really is that big of a deal.) So what is the sweet spot? Most manufactures will say around 50%. That is based on what is practical. If you wanted to never go below 30% discharge you would need two batteries to produce the same amount of power and as batteries are big and heavy you don?t want to fill your boat with batteries.
At some point we will all have to test a battery to determine its condition and the best way to do it is with a hydrometer. Unfortunately a LOT of batteries sold as Marine / RV deep cycle batteries are sealed making testing with a hydrometer all but impossible. One of the big reasons I don?t buy sealed batteries, that and the fact I like to make certain they have the proper amount of electrolyte in them. With a hydrometer not only can you check the batteries over all condition, you can check each battery cell. To use a hydrometer you will want to fully charge the battery, let it sit 10 to 12 hours and then test. One of the key things you are looking for is that all the cells read the same or within .05 variance. If they aren?t all relatively close in specific gravity the battery is on its way to failure and should be replaced in the near future. In Figure 3 you can see what the charge state is by measuring the specific gravity of the electrolyte.
Charging rates are also very often misunderstood. Faster is NOT better! The typical deep cycle battery should be charged over 8 to 10 hours. Charging them faster shortens their life just as surely as discharging them too fast or too deeply will. So what size charger should be used? As noted above, manufactures recommend not going below the 50% discharge level so we will assume that is as far as we push them. Now let?s say it?s a 100 AH (Amp Hour) battery so we divide the AH rating by two (50% discharge) and then divide that by the time it takes to charge them, say 8 hours.
100 divided by 2 = 50 divided by 8 = 6.25. In this scenario the charger should NOT produce more than 6.25 amps at its maximum output.
If you have a bank of batteries, (say four) you add their AH together and do the same math. (Four 100 AH batteries)
4 x 100 = 400 divided by 2 = 200 divided by 8 = 25. In this scenario the charger should NOT produce more than 25 amps at its maximum output.
Yes I know, someone will chime in and say they charge them faster. Those people also replace them more often than they need to. ;-) Charging at a higher rate than is needed only wastes electrical power, overheats the batteries, and shortens the batteries life span.
There is an exception, this formula is for ?flooded? batteries. Gel batteries MUST be charged considerably slower and if you use them you should get the manufactures specifications on charge rates. In a flooded battery (standard lead acid) if bubbles are formed during charging they float to the top of the acid and pose no problem. In a Gel battery the bubbles can?t float and you end up with voids in the gel and that reduces the contact with the plates and you end up with a poorly performing battery.
Discharge rates can also be confusing due to how the AH rating is computed. There are two variables when considering AH ratings, the amps you can draw and what period of time you can draw them. The vast majority of ratings for AH are based on 20 hours of use / discharge. What does that mean in the real world? Lets again take a 100 AH battery as our average battery. In THERORY that battery should produce 100 amps for 1 hour. In the real world it comes nowhere close to that. At 100 amps draw that battery would be lucky to last 20 minutes. Again, the industry standard is to calculate AH based on 20 hours of discharge. That means you could pull 5 amps for 20 hours. Keep in mind that is with a new battery in perfect condition, as they age and go through the charge / discharge cycle that AH rating falls off. Most manufactures will provide a discharge rate (curve) based on load and time and it?s worth reviewing before buying! In my opinion there are two companies that stand head and shoulders above the rest when it comes to deep cycle batteries providing the most power for the longest period of time, Trojan and Crown. Unfortunately they charge a price commensurate with their product.
Maintenance chargers are something I?d like to see kicked to the curb. They work well enough on NEW batteries that are in top condition but reduce the life span as the batteries age. How so? Go back to figure 1 and you see that a fully charged battery should have 12.7 volts while a battery with only 90% charge has 12.5 volts. That is only a difference of .2 volts. Now let?s say your batteries are a year old and have been through 50 charge / discharge cycles. It may well be that they can only get to say 90% or 95% of the full charge they could obtain when new. The problem is, a maintenance charger has no way of determining if your battery is old or new and it will continue to try to charge it to 100%. As it can?t, it just sits there cooking the battery. In my opinion you are MUCH better off having a 2 amp trickle charger you put on the battery for a 3 or 4 hours once a month during non-use periods.
