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One common question concerning deep cycle batteries is how low can they be safely discharged.
If you keep over-discharging a battery more than what it’s rated for, you can cause it to have a lower cycle life.
The battery’s cycle life is an important term used to describe deep-cycle batteries.
One battery cycle involves discharging the battery by a certain amount and then recharging it to full the amount.
The battery’s cycle life is the number of charges and discharges that a battery can complete before losing performance.
So, in other words, over-discharging a battery more than what it’s rated for, instead of having 10 or 12 years out of your battery you might only be able to get 1 or 2 years out of it.
So in this easy-to-read post, let’s look at how low you can really discharge a deep cycle battery without damaging it. And we’ll also look at other factors that will determine the cycle life and the performance of a deep cycle battery.
Discharging a Battery vs Battery Type
First, you need a true deep cycle battery because they’re made to discharge to very low voltages safely.
While batteries such as marine or car batteries are great for high-cranking amperage (where you want a lot of discharge to make a lot of electricity fast), these batteries can only depth of discharge (DoD) only 2 to 5 percent.
Depth of Discharge
The amount you discharge a battery is called depth of discharge (DoD).
For example, if you have a 100 amp-hour battery and you use only 20 amp-hours, you discharged your battery 20%. This means your depth of discharge is 20% (and your state of charge or SOC is 80%).
If you have a true deep cycle battery, you can safely discharge it 80% down. In other words, you can go down to 20% capacity and the battery won’t have that much damage.
However, even if you have a true deep cycle battery (such as lithium or an AGM sealed lead acid battery), the life cycle will decrease if you discharge it excessively.
So, if you discharge a battery to the point where it only has 20% capacity left every day, you’re probably going to get about 500 cycles out of the best battery.
But, if you discharge a battery to the point where it always has at least 50 to 70% capacity left every day, you’ll get thousands of cycles out of the same battery.
When choosing a deep cycle battery, I recommend just getting an AGM sealed deep cycle battery or a highly-rated lithium battery (lithium-ion or lithium iron phosphate).
AGM sealed lead-acid and lithium batteries will cost more initially, compared to other batteries, but when you look at the depth of discharge (DoD) and you look at their cycle life, they’re actually much cheaper in the end.
Discharging a Battery vs Charge Rate
The next factor to consider is the charge rate of the battery.
If you charge a battery faster than what it’s rated for, it’s going to give it off as excess heat. This will ultimately degrade and permanently damage your battery. So, what you want to do is charge the battery at what it’s rated for.
If you use a tiny battery with a large solar system, what’s going to happen is you’re going to quickly over-discharge the battery and then your solar system is going to charge your battery too quickly. This can cause damage to your battery.
For example, if you have a thousand watts of solar panels on your roof, but you have a small battery bank, it’s going to charge in about 20 minutes to an hour. That excess electricity is going to be given off as heat because you’re not using it.
This is why I recommend you get a large battery bank (more than one battery for a solar system).
If you have a smaller solar system with a large battery bank, it will slowly charge throughout the day into a full complete charge cycle. And, that is ideal.
When you do fast charge cycles all the time it’s going to heat and cool, heat and cool. And in the end, it’s going to degrade your battery. So what you want is a very large battery bank so that all the batteries can charge at an ideal rate (which is a slow, low current).
Keep in mind this all changes depending on the ambient temperature. When it’s cold outside, you have lower amperage potential but you have a longer lifespan of the battery (you have more amp-hours). When it’s hot outside, it has a higher discharge rate but you have fewer amp-hours or capacity.
How Big Your Battery Bank Should Be
To give you a rough estimate of how big your battery bank should be, here’s a simple trick. For every single 100 watt panel you have, multiply that by 6 to get how many amps your battery bank should have.
For example, if you have 4 x 100-watt solar panels on your roof, you’re going to need 24 amps (6 x 4 = 24 amps).
With 24 amps, you can safely charge most deep-cycle batteries.
For example, if you have a 30 amp AGM deep-cycle battery, then you can easily use it with a 400-watt solar system.
Discharging Lead Acid vs Lithium Batteries
If you have a lithium battery and you occasionally discharge the battery 100%, it doesn’t mean that you will be stuck with an inefficient battery. However, discharging a battery too much too often (beyond 80% DOD) does reduce the life of the battery.
On the other hand, most lead-acid batteries experience significantly reduced cycle life if they are discharged below 50%.
The charge and discharge rate of lithium batteries tend to depend on the type and size of the batteries (how many cells are in the battery). If your lithium battery gets really hot and you keep charging it, it’s going to degrade really fast (a lot easier than an AGM battery).
Charge Controller vs Discharging Batteries
If you have a solar power system with a cheap pulse width modulation (PMW) charge controller, switch it out for an MPPT charger controller immediately if you want your batteries to last.
A PMW charge controller is pretty much just an on and off switch and they’re horribly inefficient.
Discharging a Battery vs Voltage
So let’s look at discharging a battery and how low the voltage can get before you start doing some damage.
Nominal Voltage of a Battery
What a lot of people don’t know about batteries is that when they see something like a 6-volt battery, a 12-volt battery, or a 24-volt battery on a battery label, what they’re actually looking at is the nominal voltage of a battery.
The nominal voltage is the number of volts assigned to any particular battery.
Nominal is simply a number used to conveniently assign the rating of a piece of equipment (like a 12-volt battery) to help match it up with other pieces of equipment (like a 12-volt solar panel, 12-volt charge controller, and a 12-volt appliance).
But did you know that most batteries actually have a voltage that’s slightly higher or lower than what you see on its label? For example, a 12-volt nominal battery could have an actual operating voltage of 12.9-volts or maybe 11.5 volts.
Manufacturers produce batteries with the nominal voltage on the label because making batteries that have an exact number of volts is amazingly difficult. Even batteries coming from the same production line can fluctuate in voltages.
Similarly, a 12-volt nominal solar panel might have an actual operating voltage of 17-volts or 13-volts.
The actual operating voltage of the equipment found in a solar power system could be widely different from the nominal rating. Again, the nominal voltage is just to help us match the equipment that will play nicely together.
Actual Operating Voltage of a Battery
In order to keep our batteries lasting a long time, we have to focus on the actual operating voltage of a battery.
First, it’s a good idea to get some kind of battery monitor (Amazon) to make sure the voltage of your battery doesn’t go too low.
I really like the battery monitors that give you a warning when the voltage is too low. Some really good lithium batteries will have an integrated BMS (Battery Management System) that will beep when the voltage is too low.
Here’s a table of how low you can safely discharge a battery:
|State of Charge
You can discharge a lithium battery 100%. But with a lead-acid battery, you really should only discharge it to 50% before you start negatively affecting the battery’s lifespan (although this varies depending on the battery).
Personally, with AGM lead-acid batteries, I just discharge down to 12.4 volts so I can get a very long life cycle. But you can safely discharge AGM batteries to about 12.2 volts (you’re not at 50% until you’re at 12.1 volts).