Your battery bank will likely be one if the most expensive components of your “off the grid” energy system. They are also the only piece of the system that will need replacing on a regular basis. Every extra day you can squeeze out of your battery bank will reduce the overall cost of your renewable energy system.
Deep cycle solar batteries have lead plates (and other materials) immersed in electrolyte. The more surface area on the plates and the more electrolyte, the more capacity the battery will have. Larger deep cycle batteries will have more plates, thicker plates and more electrolyte to give the batteries more capacity.
Our job as battery owners is to preserve the surface area on the plates. If we can keep the plates clean and in good shape, we can extend the capacity and longevity of the battery.
PLEASE DON’T GET OVERWHELMED!
There are many tips below but some are a one-time deal such as setting the correct bulk voltage. Once it is done once, you will not need to think about it again.
Some are more complicated or require your attention on a regular basis. The more you work at it, the longer your batteries will last. However, if you work many hours per week for a few years to maintain your batteries, it might make more sense to buy new batteries as needed and spend the extra time with your family.
Your first battery bank will take the most abuse and last the least amount of time. As you get better at battery maintenance and monitoring your power system, you will be able to make your batteries last longer.
We always recommended an inexpensive battery bank if you are new to off grid living. Learn more about the best batteries for solar newbies here.
Here are many methods of making your batteries last longer than even the manufacturer’s specifications:
Do not have more than two parallel strings.
When you are building a battery bank, you will often have to wire your batteries in parallel to get the required amp hours. Most experts agree that paralleling battery strings is not the best idea. Having two parallel strings will not cause any issue but three or more will result in unbalanced batteries as some will take most of the charge and discharge, while others will sit almost dormant. If you must use three or more strings in parallel be sure to tie them together as shown in the diagram and equalize a little more frequently than usual. (Click on the picture to zoom in.)
Do not discharge your batteries any more than 50%.
In fact, the less you discharge your battery bank, the longer it will last.
Every time a battery is discharged and re-charged, it is referred to as a cycle. Every time a battery is cycled it wears. The deeper the cycle, the greater the wear. If your batteries were designed to cycle 500-800 at 80% depth 0f discharge (meaning 20% of the capacity left in the battery), they may last 2000 or more cycles at 20% discharge (80% still left in the battery). Cycling a battery actually removes a tiny amount of material from the plates as well as causes a buildup of sulfate on the remaining plate material. Both of these actions decrease battery capacity and longevity.
Always keep your batteries filled with distilled water.
If you are using flooded lead acid batteries in your power system it is very important to keep them topped off with distilled water. There are two reasons. First, the plates must always be covered with electrolyte. Second, if water is not added, the electrolyte will become too concentrated leading to premature battery failure. If the battery’s plates are exposed to the air for even a day or two, they will be damaged. If this happens, fill them asap and perform an equalization charge to mix the electrolyte.
Only use distilled water. Using tap water will add dirt, dissolved solids, elements and minerals to your electrolyte.
Make sure the bulk/absorption voltage is correct.
ALWAYS follow the manufacturer’s instructions when setting up your solar charge controller, wind turbine controller, inverter charger or other charging equipment. The settings you need to be concerned with are the bulk/absorption voltage, float voltage and equalization voltage. The manufacturers test their batteries regularly as well as receive feedback from trusted solar installers to come up with the best voltages for your battery bank.
If you set the voltages too low, the batteries will have a build up of sulfate on their plates that will harden over time and become impossible to remove. If you set the voltages too high, the batteries can be overheated warping the plates or have more plate material removed than necessary.
Get a battery temperature sensor for your solar charge controller, wind turbine controller, and inverter battery charger.
Temperature has a huge impact on battery voltage readings. A cold battery will have unrealistically low voltage readings compared to a warm battery. As battery temperature increases, so does the at rest voltage. A high voltage reading from a hot battery does not mean it is full, it just means it is hot. A battery temperature sensor will make the necessary adjustments to charging voltages as the temperature changes.
Let’s imagine you have two Surrette S460s in series to form a 12 volt battery bank. Surrette tells us to bulk/absorption charge these batteries at 14.4 volts. However 14.4 volts is only correct at 25 degrees Celsius (C). As the battery temperature decreases, it takes more voltage to fill the S460s. At 0 (zero) degrees Celsius (C) it will take 15.0 volts for bulk/absorption. If your controller were to charge these cold batteries at 14.4 volts, they would always remain undercharged. You cannot make these voltage adjustments manually as the battery temperature can change many time throughout the day.
Another mistake is using a charge controller with a built in temperature sensor. Although it is better than nothing, the inside of your charge controller will not be the same temperature as your batteries. Make sure you have a temperature sensor that mounts to the batteries and install it directly against the case (below the level of the electrolyte) of one of the batteries in the middle of the battery bank.
DO NOT allow your batteries to get hot.
Although having a hot battery will increase its capacity it will destroy your batteries fast. The colder a battery is, the longer it will last. In fact a typical deep cycle battery at -30 degrees C will have a 60% longer lifespan than at 25 degrees C.
We have already mentioned that batteries are rated at 25 degrees C. Every 8 degree C rise (over 25 degrees C) will cut the battery lifespan in half. Just by having your battery bank at 33 degrees C you will only get half the rated cycles.
If you were thinking of installing your batteries in an insulated box please be cautious of overheating.
Never charge a battery that is over 35 degrees C.
As we have discussed, the colder a battery bank is, the better its lifespan will be. However the real damage is done when a battery is actually being charged when it is hot. If your batteries are over 35 degrees C do not allow them to be charged. Do whatever is necessary to cool them first.
Do not try to get away with the minimum amount of batteries.
Trying to save money in the beginning by installing the bare minimum battery bank will always end in disaster. Your system will never perform well. Your spouse will always wonder why your electricity is always turning off. Having an undersized battery bank will mean deep cycles and very limited battery life. In the short term, you will save some money but in the long term it will cost you when you have to buy a second battery bank. Almost everyone makes this mistake. You will not!
Cold batteries are fine.
You do not have to worry about your batteries getting cold. They do not mind at all. It is true that capacity will be reduced but as we discussed above, the battery bank itself will have a lengthened lifetime. Even if your batteries are located in a cold environment, they are likely staying warm enough as batteries produce a lot of heat while being charged and discharged.
Make sure your absorption time is correct.
Bulk charging will bring the battery bank to a bout 80% state of charge (SOC). The absorption charge is the time spent charging once the voltage has reached the bulk voltage, All three stage chargers allow you to program how long you want your absorption charge to be. The time setting for absorption charging is critical to battery longevity. If this is not correct, your batteries will never reach 100% state of charge causing sulfation (if setting too low) or plate wear (if setting too high). What is bulk charging versus absorption charging?
Do not charge at more than 13% of your battery’s C20 AH rating.
Charging batteries at too high of a current will overheat them and caused damage by warping the plates and removing plate material. The best way to decide how much of a current your batteries can handle is to find the battery’s C20 amp hour rating (What is a C20 rating?) and multiply it .13 (13%). This will be the maximum charge rate you should put in your batteries.
For example: You have 4 Trojan L16s in series to make a 370 AH 24 volt battery bank. You just bought a brand new shiny Magnum Energy MS4024-PAE inverter that includes a 105 amp battery charger. Using your 5000 watt cheapo gas generator, you could easily force the full 105 amps into the battery bank. But should you?
Using our new formula we calculate the following:
C20 AH RATING x .13 = MAXIMUM CHARGE RATE
370 x .13 = 48.1 AMPS
MAXIMUM CHARGE RATE = 48.1 AMPS
Realistically you should never force more than 48 amps into this battery bank. This includes any wind power, solar power, water power and/or generator power. All charging sources totaled should not be more than 13% of the battery bank’s C20 rating. If it happens on occasion, it will not likely cause noticeable damage as the current will automatically limit itself as the bulk voltage is reached. But chronic over current into your battery bank will cause irreparable damage.
Please note: Some battery manufacturers only allow 10% of the C20 AH rate for the bulk current setting.
When purchasing your new bank of batteries you should also get a quality battery hydrometer that has built in temperature compensation. Knowing the specific gravity (S.G.) of the electrolyte in your batteries can tell you the health of your batteries and if your charging them properly. All battery manufacturer’s provide the recommended S.G. for your batteries. If your readings are high, the battery will have increased capacity but decreased lifespan. After equalizing your batteries, the S.G. readings of all of the batteries should be the same. If they are not, it could mean your equalization was not successful. It could also mean you have a bad connection or bad cell or cells in a battery.
When the batteries are full, the S.G. readings will be the highest. When they are discharged, they will be the lowest. After S.G. monitoring, you will get to know how your batteries perform compared to their specific gravity readings. Specific gravity readings that drop unusually fast (while being discharged), can be a sign your batteries are sulfated.
Do not allow corrosion on your posts, connections and cables
Just like your car battery, your solar batteries can have a buildup of corrosion on the terminals, cables and connections. Corrosion, being less conductive than copper, can make it harder for current to pass thru your battery cables and into your inverter, solar system or wind turbine etc… The other problem with corrosion is that it can build up from the positive to the negative post making a connection between the two posts.
Although the corrosion is less conductive than lead or copper, it is still conductive allowing current to pass from one terminal to another. Enough corrosion can drain a battery within hours.
Always keep an eye on your battery bank.
Every so often take a look inside the battery box and see how things are going. Is there any leakage? Does anything look unusual? Does it smell unusual or burnt? Is there excess liquid on the tops of the batteries? Any corrosion?
Feel the tops and sides of your batteries. Are there any hot spots? Does one battery feel hotter than the others?
Look at and feel the battery cables and their crimped/soldered lugs. Are they hot? Do they look like they have been hot in the past? It is very common for the lugs crimped on the end of a cable to begin to fail and get very hot. They can get so hot they melt the battery terminals off.
The last check is to remove all the battery caps and check the water levels (if you have flooded lead acid batteries). While you are in the battery, look at the plates and the electrolyte. Does it look clean? Does it look the same as all the other batteries?
It may seem like a lot of work to take good care of your batteries but it can be worth it due to the extra money you will save by not needing to purchase a battery bank until you get every kWh out of it.
When you consider that most solar equipment will last 20 years or more, taking the extra time to care for your batteries is a good use of your resources and time. It will also be fun to brag to your friends about long you made your battery bank last.