How Long a Deep Cycle Battery Holds a Charge
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All deep cycle batteries are rated in Amp Hours (or AH). This is a rating of how much amperage can be drawn from your battery before it gets fully discharged (and be around 10.5 v). 100AH battery (for example) should provide 1 amp of power for 100 hours or 5 amps for 20 hours (with more drawn amps equivalent to proportionately fewer hours):
100 AH (battery) / 10 amp (device) = 10 hours (usage time)
If you pull 30 amps from your 100 AH battery in about an hour, it means that you just used 30 AH (amp hours) and you only have 70 AH left. Having said that, you definitely do not want to draw all amp-hours completely, half of the battery capacity is usually a good number.
** Please note that older batteries do not have the full AH capacity that is stated on them.
This is how you can calculate your usage correctly:
- Wattage and usage time. Find out how many watts your single device will be using and for how long.
- List of devices. Make a list of all your devices with their specific wattage and time rating.
- AMP rating. Convert wattage into AMPs. If you will be using a 12-volt system, simply divide your wattage by 12.
- Daily usage hours. Calculate how many hours in a day you will be using your battery by adding the number of hours and minutes (1/60*Mins)+Hours. Example: (0.016 * 15min) + 2 hours = 2.24 hours.
- Daily AMP usage. Multiply the number of usage hours (2.24 in our example) by your device’s AMP rating.
- Total daily AMP usage. Add this AMP usage rating per day for all your devices.
- Calculate battery hours. Divide the battery’s useful Amp Hours (whatever is left in a battery) by your total daily AMP usage to find out how many hours you can power your calculated load on this battery.
** Your battery (or batteries) “useful life” may or may not be their full capacity. If they are down 20%, you should calculate it accordingly (you can use an electronic measuring device like a voltmeter to get voltage ratings which could give you approximate battery capacity).
Here is a 12 Volt battery example:
|Voltage||Capacity||100 AH||180 AH|
** If you are not sure how many AMPS your device consumes, simply divide WATTS by VOLTS to get this number. Example: 75-watt appliance running on a 12-volt power source will use 6.25 amps (75 /12 = 6.25)
Here is an example of 12-volt battery usage with different size loads. If you want to stay on the safe side, cut a number of hours in half in order not to discharge more than 50% of your battery:
|AC load||AMPs (watts/12v)||100 AH||210 AH|
|50w||4.16||24 h||50.4 h|
|100w||8.33||12 h||25.21 h|
|200w||16.66||6 h||12.60 h|
|500w||41.66||2.4 h||5.04 h|
|1000w||83.33||1.2 h||2.52 h|
|1500w||125.00||0.8 h||1.68 h|
|2000w||166.66||0.6 h||1.26 h|
The common lifespan of a deep cycle battery (according to some internet sources) is:
- 1 to 6 years for Marine deep cycle battery
- 2 to 5 years for Gel deep cycle battery
- 4 to 8 years for AGM (Absorbed Glass Mat) deep cycle battery
- 4 to 8 years for Flooded deep cycle battery
- 4 to 8 years for L-16 type deep cycle battery
- 7-18 years for premium deep cycle battery (like Rolls-Surrette)
- 10-20 years for industrial deep cycle battery (like Crown and Rolls 4KS series)
But to be real, these numbers most battery owners find it quite difficult to accomplish due to constant misuse and abuse of the battery. Anything from overcharging and undercharging will not do your battery any good and it could go bad within a year or so.
On the other hand, we have to be happy! If we don’t use the batteries according to our needs, then what’s the point of having this RV (for example) in the first place? Some people feel that using their batteries to the full capacity (as long as it doesn’t completely discharge) makes more sense to them since replacing a battery may end up being cheaper than looking for the RV park.
** The longevity of batteries is measured in cycles and sometimes if you read the fine print, it will mention at what level of discharge they are advertising their lifespan. It could be as low as 5% discharge that gives you 20-year longevity or 5 years at 50% discharge.
Life “cycle” of a battery
When we talk about cycles of the batteries, that usually means a measurement of complete discharge/recharge sets (or cycles). When the battery manufacturer talks about how many “cycles” a battery can handle, always pay attention to “how deep” is the discharge that they are talking about.
Low discharge is definitely good for the battery and it really prolongs its life, but it’s not what necessary we buy it for! It is great for the optimal performance of a battery, but it has nothing to do with real-life usage.
Some manufacturers specify a rating called: a “Depth of Discharge” and that simply means the % of battery charge that you can use before recharging it. Manufacturers usually guarantee a certain amount of years that you can use a battery for if…
… it falls within range of specified amount of cycles and DOD (or Depth of Discharge).
The more “cycles” the battery can handle and a deeper level of discharge is always of a good value. But this is a fluctuating number since the life span of your battery will be about the same if your amount of cycles is less, but you discharge it deeper, and if you discharge it less, but you cycle more.
** This is just a possible example, refer to your owner’s manual or manufacturers ratings
Depth of Discharge (DOD)
It is true that the more times we charge and discharge a battery, the shorter will be it’s lifespan. This also applies to discharging the battery entirely.
But this is probably why you’ve got a deep cycle battery in the first place! A deep charge battery allows you to use ALMOST all the energy that is stored in your battery safely, as long as you follow the manufacturer’s guidelines.
50% to 80% DOD is usually recommended by most manufacturers for a deep cycle battery. To prolong the life span of this type of battery, users generally recommend no more than 50% DOD.