To my knowledge of a battery drops under 10 volts on start then it is unhealthy and need recharging/replacing.

However I have just read another answer where it states a battery with a dead cell can fall to 10.5 volts under load.

So how do we decide 10 volts is enough??

  • I think you misread or misunderstand the intent of the other answer about 10.5vdc. If you put a load on the battery with a dead cell, then release the load, you'll see the voltage down around the 10.5vdc arena, not while under load. If yours is going back to 12-13vdc after the load is release, this is not your issue. I've never tested to see what the voltage drops to under load for a healthy battery. Nov 9 '20 at 21:12
  • @Paulster2 Oh yes, seems I have misunderstood it but not anymore! Nov 9 '20 at 22:25
  • @Pᴀᴜʟsᴛᴇʀ2 before the days of electronic ignition, typical coil "ballast resistors" were sized so that sparks while cranking the engine were good down to 6V. The resistor was shorted out while cranking, but stopped the coil burning out when the full 12V was applied in normal engine running.
    – alephzero
    Nov 10 '20 at 3:13
  • @alephzero many were actually 9V and if the resistor failed a temorary one could be made with 6” of 14/030 wire -at least that was the quick fix from the Technical Bulletin.
    – Solar Mike
    Nov 10 '20 at 6:09
  • @alephzero - You are correct for the most part. I don't think the ballast resistor was shorted, but more so bypassed to give full voltage on start up. The reason for the ballast was so you didn't burn the points out under full battery voltage all the time. The coil could properly handle things, but not the points (IIRC). It should also be noted in this day of electronic ignition systems, under low voltage the ECU can do really wonky things. Nov 10 '20 at 17:30

First a little background information:

Good Battery

A good battery will sit around 12.6 to 12.8 volts when fully charged.

When a good battery is put through a load test equal to its rated CCA (cold cranking amps) its voltage will drop to around 9.6 to 10.5 volts depending on the ambient temperature. It will then shoot back up to ~12.6 volts once the load is removed.

Bad Battery

A battery with one or more dead cells loses around 2.1 volts with each cell that has died. A battery with 1 dead cell therefore has a voltage of around 10.5 volts, 2 dead cells = 9.4 volts, etc. But usually once one cell goes bad the battery is replaced before others die as well.

A bad battery can show a false voltage when it has surface charge, this occurs for a length of time after a battery has been charging. It can read a full voltage of 12.6 even though it has a bad cell.

However, when a battery with a bad cell is put under load, it will immediately fall well below its real voltage of 10.5 volts. Once the load is removed, it will only bounce back up to its maximum 10.5 volts.

So when is 10 volts enough?

To answer your question, 10 volts under a load test shows a good battery, especially when it immediately bounces back up to over 12 volts once the load is removed. 10 volts on a battery without load shows a dead cell and when put under load will usually fall well below 10 volts. A battery that shows a voltage of 12+ volts but falls below 10 volts under load and only rebounds to 10 volts after the load is removed is also a bad battery and likely has a dead cell.

EDIT: Starting a car is very different from a load test. The battery is under load for much less time. A battery that falls below 10 volts on startup but that consistently starts the vehicle is probably either a little under charged or is aging and has lost some of its cranking power as all batteries do over time. Putting the battery on a charger will solve the under charged issue. If it is still falling below 10 volts after a good charge then the latter issue is the case.

  • J-Rome it seems you are saying a good battery need not drop to 10 volts on load but it can drop to 9.6 volts on load so long as it jumps back up to above 12? Im frequently seeing 9.66 on startup so I guess its good. Many of the vids I have seen basically say if it falls below 10 volts then it’s bad but based on your answer that is incorrect the figure should be 9.6. Why do I think some mechanics are suggesting 10 volts? Thanks – James Wilson 9 mins ago Delete Nov 10 '20 at 17:39
  • @JamesWilson In my experience of load testing hundreds of car batteries when performing the test you hold it under load, equal to its rated CCA for 15 seconds, under these conditions a good battery will fall in the range I mentioned (9.6 - 10.5 volts). Startup is a very different scenario with the battery under load much less than 15 seconds. If your battery is falling below 10 volts but can consistently start your car without issues, then I would say there is a good chance it is either a little under charged, or it is aging and losing some of its cranking power.
    – Mr. Spock
    Nov 10 '20 at 17:42
  • I will edit my answer to include this info
    – Mr. Spock
    Nov 10 '20 at 17:42

An easy but hard lesson to learn when a car battery needs to be replaced would be either in summer or freezing cold temperatures with the battery unable to power the starter and a boost is needed. Being stranded with a dead battery despite knowledge of voltage levels may not prevent the acid test in real world extreme temperatures of hot/humid summer or freezing winter when a battery is at its weakest and has served its purpose beyond its warranty date. Most batteries last all the way to the end of their warranty and promptly die. There are hints along the way. The best way to test a car battery is thru a trusted auto parts store selling car batteries, providing free battery/alternator testing whether in car or bench. Good load testers places a load as if powering a starter drawing large amperage for a timed period (15-30 seconds or more) then reading voltage. Whether computerized or analog testing, good batteries will pass while marginal ones may just need charging. A battery nearing its end of life will not hold a charge and usually a candidate for replacing. Honest auto parts stores will give accurate advice while the dishonest ones are to be avoided.

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