Making larger battery packs is often required to extend backup time or increase voltage to make certain devices work. For example, if you own solar power system or Power inverter, you can connect several batteries to them to get more juice and more run time. Communication networks and small and large servers also use backup UPS which are circuited with often times large number of batteries or larger batteries. Different kind of packs are made depending upon needs and to cut maintenance costs.
Here, I have elaborately explained how to make Parallel, Series and Series-Parallel combined battery packs (battery banks). This guide is quite helpful for newbie users who want to learn how to connect lead acid batteries (Sealed, VRLA, MF, Gel, AGM, Wet or flooded) together while circuiting them with solar power systems, uninterruptible power supply (UPS) systems, power inverters or battery chargers. Additionally, I have also discussed some frequently asked questions related to the subject in FAQs section below. Note that Battery pack is also called as battery bank and AGM and gel batteries are also known as maintenance free or dry batteries in some regions.
Batteries in Parallel Connection (Parallel Battery Pack)
In this type of battery bank, batteries are connected from terminals to same terminals of other batteries that is positive terminal (+) of one battery is connected with positive (+) terminal of other battery and negative terminal (-) of one battery with negative terminal (-) of other battery. See the diagram below for more idea:
Batteries in Series Connection (Series Battery Pack)
Batteries are connected from terminal to terminal in a way that positive (+) terminal of one battery is connected with negative (-) terminal of other battery and negative terminal (-) of one battery is connected with positive terminal (+) of other battery. See the diagram for more idea:
Different Size Batteries in Parallel or Series Connections
In Parallel
(Criteria: If batteries are labeled as similar voltage but different capacity)
In Series
(Criteria: If batteries are labeled as similar voltage but different capacity)
Batteries In Series And Parallel Combination
In series-parallel batteries combination, one pack of batteries connected in series is joined in parallel with another pack of batteries connected in series. Therefore, the overall output voltage of series packs remains the same. But charge storage capacity is increased.
1- Why batteries are connected in parallel?
Connecting batteries in parallel keeps the voltage of whole pack same but multiplies the storage capacity and energy in Ampere hour (Ah) and Watt hour (Wh).
2- Why batteries are connected in series?
Connecting batteries in series multiplies the voltage but keeps the capacity in Ampere hour (Ah) same. Energy in watt hour (Wh) increases. According to the common sense, overall charge storage capacity is also increased because there are more reservoirs available now to store charge.
3- Does connecting batteries in series or parallel increase capacity and backup?
Yes. As I mentioned above, it’s like now you have two or more reservoirs to store charge instead of one. The backup provided by the system will be increased. But you can not keep connecting batteries in series if a power device is designed to accept specific voltage. Connect them in parallel to increase backup or buy larger batteries instead.
4- Why is connecting different capacity batteries in parallel to each other is not recommended for the long term use?
Batteries with different capacity but same voltage can be connected in parallel but it is preferable not to do this. Because there are possibilities that different size batteries have slight voltage difference even though they are labelled as similar voltage on the label. This will lead to potential difference among connected batteries which means batteries with higher voltage will try to charge the battery with lower voltage which can result in heat up and destruction of that battery. Also, when different capacity batteries are connected in parallel to UPS or Power Inverter, charger of UPS might cause conflict and start behaving abnormally. To minimize such risks and troubles, buy similar capacity and voltage batteries of same brand made by the same company. Never mix different brands of batteries from the same or different manufacturers.
5- Why different capacity batteries should not be connected in series?
Never connect different capacity batteries in series to each other. If connected, smaller capacity battery will charge first but larger battery will still be empty. This will result in heating and over charging of smaller battery. In discharge mode, smaller battery will get empty first resulting in deep discharge of that battery. To make series battery bank, buy similar capacity and voltage batteries of same brand and company.
6- Can I mix old and new batteries in parallel and series?
A very bad idea to mix old and new batteries in parallel. Old batteries which have fairly worn out do not retain the voltage like new batteries. So, if old batteries are mixed with new batteries, this will shorten the life span of new batteries and damage old batteries too. But you may connect old and new batteries in series although I won’t recommend it either. Old battery may fail to attain cut off voltage which will result in overcharging and overheating of new and healthy battery as well.
7- Can I mix different types of lead acid batteries?
No. Never do it. Each lead acid battery type including VRLA, AGM, Gel, Wet or flooded has got different charge rate and different maximum and minimum tolerable voltages. Mixing them means damaging them and wasting your money.
8- Why large battery packs are made?
Series battery pack is made to multiply the voltage. This helps in decreasing the size of transformer which is used to step up the voltage. Machines which require high voltage direct current (HVDC) to operate request for the series battery pack. Parallel battery pack just increases the back up.
Safety:
High capacity batteries should be handled with care. Never short circuit them as short circuiting can cause fire and explosion of batteries. Must connect batteries correctly with UPS, inverter or solar power system. Wrong connection can damage the unit.