Can four car batteries with inconsistent voltages be used in series?
3 Answers
If four car batteries with inconsistent voltages are connected in series, their capacities will inevitably differ, so it is best not to use them this way. The detailed reasons why this is not advisable are as follows: Discharging: When batteries with different capacities discharge, one will always deplete its charge first while the other still maintains a higher voltage. In this case, even if there is a low-voltage protection function like that in an electric vehicle controller, timely protection cannot be ensured. Battery Discharge: If the battery voltage drops below 10% of the rated voltage, there is a risk of discharge. Once a battery is discharged, its lifespan is reduced by 80%. Charging: When charging batteries with different capacities, one will always reach full charge first while the other remains at a lower voltage. At this point, the charger will continue charging, posing an overcharging risk to the already fully charged battery.
I've been driving trucks for over a decade. Once I tried powering my vehicle with four used batteries connected in series, but their voltages varied significantly, which nearly caused a major accident. One battery was only at 11 volts while the others were above 12 volts. When connected in series, the entire system experienced current imbalance, and during charging, the low-voltage battery over-discharged and burned out completely. I initially wanted to save money but ended up spending more on new batteries. Now, before installing any batteries, I always use a voltmeter to check each battery's voltage, ensuring they're all around 12 volts before connecting. If series connection is truly necessary, I recommend using professional voltage balancing equipment to match the voltages. Otherwise, the risks are too high—it could lead to short circuits or even fires. Regularly check your vehicle's battery condition; don't take risks to save a few bucks. Safety always comes first—nothing is more important than that.
Back when I was an apprentice, my mentor taught me that batteries in series must have identical voltages. A difference of more than 0.2 volts will cause charging imbalance. If four batteries have varying voltages, connecting them in series makes the stronger battery bear more load while the weaker one gets overcharged and fails, shortening all batteries' lifespan. I once encountered a customer who modified an electric vehicle with series-connected batteries having over 1-volt difference—all batteries were ruined in less than a month, and the vehicle broke down. Now, I always recommend using a tester to check each battery's voltage first. If the difference exceeds 0.5 volts, avoid series connection and opt for parallel connection instead—it only increases capacity without boosting voltage, making it safer. During routine maintenance, record each battery's voltage data to help the manufacturer match old batteries or replace them, ensuring peace of mind.
