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The charging current determines how many batteries you can use with an inverter. The battery capacity cannot exceed the charging current limits, otherwise the battery will take too long to charge or not all. This applies to all types of solar inverters regardless of size.
The number of batteries you can connect to an inverter cannot be more than 12 times the inverter charging current. A 20A charger can handle 240ah battery maximum. The formula is A x 12 = battery capacity (ah). If it is a 40A charger the limit is 480ah. It can be any number of batteries as long as the total ah does not exceed the charge current limit.
How Much Current is Needed to Charge an Inverter Battery?
The first thing you have to do is figure out how much current is required. Fortunately the process are very simple.
Suppose you have a high quality 200ah battery like the BatteryJack 12V AGM. Using the formula above a 20A charge current will be enough. A higher charge current is needed for larger battery capacities. Now the question is, how long will a 200ah take to charge with this current?
To find out, divide the charge current by the amp hours (ah). In our example that’s 200/20 = 10. A 20A charge takes 10 hours to charge a 200ah battery. However inverters are not perfect, so expect an efficiency rating of 80%. An 80% efficiency means it takes a 20A charger 12 hours to charge a 200ah battery.
You can use the same formula for other inverters, but it is better to check your product guide. It should mention there its efficiency rating. Use that figure for your calculation and you will get a more accurate estimate of the charge time.
The charge time also depends on the type of battery used. Lithium-ion is superior to lead acid and charges faster though cots more. If you want to go with lithium, a good choice is the Weize LiFePO4 as it works at home, RVs and marine applications.
Battery charges also slow down as it nears full charge, so at 12 hours you might be at 80-85% only. And that is not a bad thing as topping off at 85% will not damage the battery. In fact it might be good for the long term.
Using the same 80% efficiency, a 30A charger should take 8 hours to charge a 200ah battery up to 85%. The higher the current, the less certain the results will be. Charge current is not only the factor. The batter depth of discharge (DOD) is also important. A completely drained battery will take longer to charge regardless of charging current.
Battery Discharge and Inverter Current Settings
Your inverter installation guide contains information on this topic. Some inverters may be able to handle more amps depending on usage. The longer the battery discharge frequency, the higher the battery can be. If the battery discharge is every ten days, the battery capacity can be 15-20 times the charge current limit. Remember that for lead acid batteries, the maximum depth of discharge is 50%.
For rapid discharges the times 12 the charge current is ideal. The inverter current must also be under the battery limiting current. This is to make certain the battery does not suffer damage during the charging process.
Series vs. Parallel Inverter Battery Configuration
Both series and parallel battery bank connections have the same goal, boot capacity for longer service. For this to work, the inverter direct current voltage and battery bank voltage have to match. This prevents the inverter from overcharging the battery and causing damage.
Battery Series Connection. The battery bank voltage increases in a series. It is the same as the total voltage of each battery. If there are three 12V 200ah batteries, the battery voltage is 36V (12V x 3 = 36). An inverter with a 36V can recharge these batteries. The maximum capacity is 600ah 9200 x 3 = 600).
Battery Parallel Connection. If the battery bank is connected in parallel, the battery bank capacity increases but the battery voltage is the same as each cell. Only batteries with similar voltage ratings should be used for even charging. Otherwise it could damage the battery.
If there are three 12V 200ah batteries, the bank voltage will be 12V. You can use a 12V rated inverter charger to power it. The maximum capacity is 600ah, similar to the series. The difference is the voltage because in a series connection it goes up to 36V.
If batteries are in a parallel connection, the inverter charger must supply the current needed by every battery. So if the battery current limit is 20 amps, and there are two batteries in parallel, the inverter must provide 40 amps (20A x 2 batteries). This is not the case if the battery bank is configured in a series, because all the batteries have a similar current.
How to Connect Batteries in Series and Parallel
Connect Batteries in a Series. To create a series connection, connect the battery positive + end to the negative – of the next battery. The positive = of the final battery in the connection and the first battery negative are then connected to the inverter or charge controller.
Connect Batteries in Parallel. To create a parallel connection, connect the positive + of the first battery to the positive+ of the next one. Next, link the negative – of the first battery to the negative of the next one. Connect the inverter positive and negative into the bank center point.
Connect in Series and Parallel. There are instances where both series and parallel connections are used simultaneously. Multiple batteries may be installed in a series to get the desired power. Then another set of batteries are set up in parallel. In these instances, the inverter and/or charge controller connection is done in the center of the battery bank.
Can an Inverter Run Without a Battery?
First we need to define what an inverter is. An inverter converts DC power into AC power. If you install solar panels in an RV or at home, you need an inverter to run AC powered appliances.
An inverter can run without batteries, but you will not be able to save any energy solar panels collect. it will convert DC into AC power and run your devices and appliances. But when the sun goes down the inverter can no longer function.
A hybrid solar inverter connects to solar panels and the power grid. You cannot use this off the grid as it uses the grid and the solar panels to supply energy. it is cheaper than battery connected inverters and easier to set up.
The disadvantage is you are tied to the grid. If there is a power outage you have no backup. This is why it is more practical to use batteries with an inverter in case of a long blackout. And as we mentioned you cannot even use this for RVs so you really have to use batteries with the system.
We want to get the maximum power from batteries and inverters, but at the same time we do not want to overdo it. By knowing the capability and capacity of your inverter, you can push to the limit without damage. The same goes with the batteries so you do not end up using too little or too much.