Battery Banks – Grid Sub

We recently did a piece about mixing up different wattage solar panels into a single array as you gradually ramp up your off-grid power production. While this isn’t ideal, it is a great way to get going even if you are on a tight budget. It happens that people on tight budgets might also consider starting with a small battery bank and adding on more units as time goes by. Since finding the exact same battery you started with might be hard, you might be tempted to introduce different capacity or manufacturer units to your battery bank. This could happen easily especially if you buy second hand or chase deals instead of going for specific capacity and brand. Today we are going to look into the consequences of mixing up battery capacities and if it is something you should do. Quick Answer: No, its not a good idea. You will either get less overall capacity, damage the batteries, or shorten their lifespan by not charging and discharging the different batteries to the right voltages. As always, the answer is not straightforward. It depends on a couple of things with the most important being: Battery chemistry and designIf you will be wiring them in series or in parallel Battery Chemistry and Design The chemistry and design of a battery has a huge implication on how it works. It dictates its SOC voltages, safe operation currents, and internal resistance. Mixing up chemistries and designs can have dire consequences even if the batteries quote the same capacity. Warning: Don’t mix different chemistries or designs into one bank. This means you can’t mix lithium iron phosphate with lead acid, Lithium Iron Phosphate with Lithium-ion, or a flooded lead acid with an AGM lead acid battery. If your battery chemistry and design are the same, you could consider mixing and move on to the next stage of our guide unless your batteries are Lithium technology based. All these batteries have internal control circuits (Battery Management Systems) that will either shut down or fail if the capacity discrepancy is too high. How You Will Be Wiring them You have two options when wiring your batteries. Parallel to increase current or in series to increase the voltage. Wiring Different Capacity Batteries in Series Using different capacity batteries in series isn’t practical. The lower capacity battery will vary its voltage faster since it can charge or discharge faster. If it is lead acid based, it will gas and be damaged faste. If it is lithium based, the BMS will shut it down sooner disrupting the entire system If this doesn’t happen, the higher capacity battery will never fully charge as the higher voltage readings from the smaller battery will kick the charger out of charge mode sooner. Lead acid batteries will sulfate while lithium batteries might survive. Connecting Different Capacity Batteries in Parallel You can theoretically get away with this if you design your system perfectly. After all, the only problem here will be the different batteries will charge and discharge at different rates. Warning: The batteries must be of the same voltage range. Don’t put a 6V battery in parallel with a 12V battery. The lower Amp Hour battery will take in charge faster and drain faster when in use. The bigger battery will in turn try charging the smaller battery leading to some energy loss but your system will still run. However, when charging, the lower capacity battery might hit high SOC voltage faster forcing the charge to go int float mode. You can go around this by including a voltage monitor and relays to control the different capacity batteries. When charging, the voltage monitor will disconect the smaller battery once it reaches its fully charged voltage leaving the bigger battery to keep chargingWhen discharging, the same system will disconect the the smaller battery once the voltage difference between it and the bigger unit is high and give it the system some sense of balance This is a bit complex and costly to implement. It’s not worth it unless you are getting the different capacity battery free or at a throw away price. Verdict: Should I Mix Different AH Batteries? No. You shouldn’t. It’s good practice to ensure that all the batteries in your bank are as matched as possible. This means: Ensure that they are the same capacityEnsure that they are from the same manufacturerBuy them at the same time to ensure they have the same wear and tear and perhaps the same manufacturing tolerances What if I Salvage My Batteries Some people are lucky enough to salvage their batteries from: Commercial power backup systemsOld electric carsPrevilidged people who want to upgrade If you fall in this category, you have a couple of options If Lead Based, Go Parallel If the batteries you get are Lead Acid, connect them in parallel. You could either add an elaborate circuit to keep them balanced or just let them be. They will wear out faster but you spent next to nothing getting them If Lithium, Disassemble Them and Build Bottom Up Most lithium chemistry batteries consist of individual cells wired in series and parallel to achieve different voltages and AH capacities. When salvaging, your best shot is carefully dismantling the batteries, testing individual cell capacities to find matching units and combining them into a new battery with an aftermarket battery management system. Disclaimer: Dismantling and working with Lithium batteries could be dangerous. Do your research well and stay safe before attempted. We are not liable to any damage or injuries arising from your undertaking of such a project.