2
\$\begingroup\$

I need a battery for low power inverter application but it seems that it's not easy for the following reason:

If i need 100 W output from 12v battery then i need to pull about 10A out of it(considering the losses). So if i need it to work for about 2 hours then that's 20Ah and considering the discharge rate of batteries then the final battery rate will be higher than 20Ah.

The problem is it's hard (for me) to get a battery with that amp rating with a good price and the only solution(as it seems) is to use a car battery since it has a high amperage capacity with a reasonable price.

Is a car battery is a good idea for this application?

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Car battery will work but have a relatively short lifetime. Car batteries are designed for "float " operation = being kept near full charge most of the time. If you want to regularly discharge a battery by a substantial % of its total capacity you need a "deep discharge" battery if you want reasonable cycle life. If you do use a car battery try to use as large a ine as you can afford so its %capacity discharge is low. Try to recharge it as soon as possible after discharge. \$\endgroup\$
    – Russell McMahon
    Apr 6, 2017 at 13:24
  • \$\begingroup\$ A "deep cycle" battery would be better than a car starting battery, because it is designed to withstand deep discharges, while starting batteries are designed to deliver high currents, then be immediately recharged. Deep cycle batteries may also be called "Marine" or "RV" batteries. \$\endgroup\$ Apr 6, 2017 at 15:48

2 Answers 2

4
\$\begingroup\$

Only you can decide if a car battery is a suitable solution for the application. Is it a workable solution from a technical standpoint? Yes it will work but there are factors that you do need to consider to determine suitability.

  1. Car batteries are heavy and large size.
  2. Lead acid car batteries are not usually sealed so there is danger of acid spill or escape.
  3. Car batteries can source lots of current and if there is accidental shorting without use of adequate protection devices the massive current can do a lot of damage.
  4. Charging lead acid car batteries leads to hydrogen gas production which can be dangerous if done indoors and/or in a confined space.
  5. Connection of low current load wires to a car battery can be somewhat of a pain as regular battery terminals do not normally accommodate small wires.
  6. You will have to evaluate the discharge rate versus available charging rate to determine if the down time duty cycle of the battery is too large for your application.
  7. If usage of the battery capacity is done over a long time period there may be self discharge loss of overall capacity to consider.
\$\endgroup\$
3
  • \$\begingroup\$ What about "car batteries are designed to operate at high currents for short periods" ? I was warned about this issue many times but i don't know how correct it is.. \$\endgroup\$
    – iMohaned
    Apr 6, 2017 at 11:30
  • 1
    \$\begingroup\$ Car batteries are optimized for relatively high current usage and corresponding high current charging from an alternator. There can be issues of the battery chemistry not working as designed when used at low current. This could result in water usage or excessive sulfate buildup on the plates in the battery. That said I have deployed car batteries for low current applications (even used ones) and had them serve my needs. I generally keep one in my garage that I charge up every 6 months or so. Its usage is reserved for connection of a large 12V spot (continued) \$\endgroup\$ Apr 6, 2017 at 11:43
  • \$\begingroup\$ (continued from above) lantern that is very handy when working on something in a dark garage when there is a power outage or I have to turn off the main breaker in the power panel. This type of lantern is usually equipped with a cigarette lighter plug which I cut off and added on a pair of replacement battery charger clamps. The car battery idea gives me the possibility of hours of lamp usage if needed. \$\endgroup\$ Apr 6, 2017 at 11:48
0
\$\begingroup\$

It is not the best option to use a car battery for the following reasons: -

  1. Safety:- Car batteries are partially sealed batteries irrespective of whether they are flooded or AGM type. Thus they can release Hydrogen gas, Sulphur Dioxide gas and Oxygen gas when they are charged or discharged or even allowed to sit idle. This combination of gases is ideal to trigger a massive explosion if there is a spark. Especially when an inverter is connected, there are huge sparks due to the presence of Capacitors inside the inverter. Thus, a partially sealed automotive battery cannot be used indoors at any cost.

  2. Not for the Purpose:- Even though an automotive battery and a deep cycle battery have the same rating in Ampere-hours (Ah), they are definitely not the same. The automotive batteries have thin plates that are meant only for sudden surge currents and thus only for automotive ignition. Thus, a car battery will not give the same amount of backup with the inverter nor will give the same amount of efficiency while being recharged.

Conclusion:- The best option is to get a sealed deep cycle battery. You can use a gel acid battery or a Valve Regulated Lead Acid (VRLA) battery, both come under the Sealed Maintenance Free (SMF) battery type. These will recharge efficiently and will also discharge efficiently delivering their full capacities and will be really ideal for the inverter use and indoor use.

Although it is good to discharge Lead Acid batteries to only 50% of their rated capacity to prolong lifespan, it is still not harmful to discharge them completely.

I recommend getting a good quality deep cycle battery and good quality smart charger that will help recharge the battery.

If the setup is being used with a solar array, I recommend getting a good quality MPPT solar charge controller.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.