I placed the CTEK multi US 7002 battery charger on a regular lead acid 12 volt car battery. I made sure the leads were correctly connected to the proper terminals on the battery. I set the CTEK to "normal" mode. The battery charger showed an error code 0 (!).

I then took an older Sears car battery charger and attached it to the same lead acid battery. The charger shows the battery about half charged and it appears to be charging the battery normally.

So, I took the CTEK to another car I have that has an AGM battery. I set the CTEK to "snowflake" mode. It is charging the AGM battery perfectly fine.

What gives?

This morning I put the Sears battery charger back on the lead cell car battery. It appears to be charging normally. The CTEK shows a fully charge condition on the AGM battery.

After the lead cell battery shows fully charged, I am going to place the CTEK on it again and hopefully it won't show an error code.

Some smart chargers don’t like very discharged batteries. The old Sears charger doesn’t care so much what the voltage in the battery is, it will deliver the current needed to bring it to the correct level of charge.

Michael

That is what I'm thinking. I believe I'll keep the old Sears charger around for a few more years......

My fancy "microprocessor controlled" Sears charger won't even come on if there is less than 8 volts at the leads and can be fooled by a battery that has a false charge. I'm sure there's a correct term but it escapes me at the moment. By false charge, I mean that it will come up to 14 volts pretty quickly but isn't really charged, it will drop right off under load. I'd keep the old school charger around, sometimes smarter isn't better.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

The trick to "fool" the newer chargers to recognize a completely dead battery is to hook a charged battery to the terminals and begin the charging process. Once the battery comes up to the minimal voltage amount the charger will take over and you can remove the auxiliary battery.

The old rectifier type chargers just slam the juice in there with no feedback required but no regulation of current. They are rough on AGM batteries and can quickly ruin them.

If your battery voltage drops quickly under a load then you have a dead or shorted cell. No amount or type of charging will amend this. Replacement time.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Thank you for the advice, Mike. I believe the lead battery may be end of life in which case my plan is to replace with an AGM.