Two Optima Batteries to start my 1916 48B

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    Hooked two 6v Optima batteries in parallel. Gives me a lot of amps at 6v. I hooked up one NOCO Genius 1.1 Amp charger and it was unable to get a full charge, so I hooked up two of the Genius chargers and it gave a full charge in about 24 hours. I would imagine that one Noco Charger should maintain the batteries.

    Don’t see any downside?


    ONE Optima turns over my 1918 48-B-5 very nicely. I run ONE in my Series 80. I run TWO in parallel in my 8-cylinder Pierces primarily for the reserve capacity during nighttime driving. Optimas have about 100 amp-hrs (AH) reserve capacity each. The Group 3 wet cell battery originally furnished in 8-cyl cars had 140 AH and the Group 4 in 12-cyl cars had 165 AH.



    The one charger should keep the batteries topped off. It would be interesting to separate the two and check the “open circuit” voltage on both to make sure they are at approximately the same voltage level. You want to parallel two batteries that are about the same, or you could wind up creating a circulating current between the two batteries.

    Also, you should check them separately once a season or so to ensure one hasn’t failed. I had that issue with a diesel pickup; couldn’t figure out why it wouldn’t start well with two one year old batteries. One had a shorted cell, and pulled the other down to 10 volts. New battery installed, problem solved.


    I recently bought a NICO 3500 Genius and have been happy with it. The NICO 1100 is only 1.1 amp and the NICO site says it is for batteries of up to 40 amp-hour capacity. The Optima is 50 AH, so if fully discharged it would take on the order of 50/1.1 45 hours to fully charge a near dead battery – 2 days. A bit longer because the NICO has sophisticated algorithms to vary the charge rate to optimize the charge.

    Charging 2 optimas in parallel would require twice as long and would exceed the stated size limitation.

    The larger NICO 3500 is 3.5 amp and is rated on their site for batteries up to 125 AH. I still use conventional group 3 batteries (I have had them last 5 to 6 years) of 140 AH which is a bit larger than the stated limit. Theoretically at maximum charge rate it would take 40 hours but actually takes about 4 to 5 days to charge a nearly dead battery and will often croak in the final optimization stage. Not a big problem, I think this charger is great.

    I think charging two batteries in parallel may throw the little electronic brain of the charger off since it is trying to adjust charge rate based on the condition and state of charge of the battery. Two batteries are not going to be identical. May work, but maybe not as well as charging separately.

    Before I bought the charger I asked NICO if it would still work even though my battery was a bit larger than 125 AH:

    “Thank you for contacting NOCO support. It’ll still work on it. The further outside of the Amp-Hour rating of the charger the battery is, the harder it is for the charger to complete the charge. If the batteries are only slightly outside the rating, then it will only be a matter of a longer charge time, but the larger the battery the more difficult it is for the charger to complete the charge cycle. This is because the charge steps the charger uses. In the final steps of charging, the charger will continually lower the current used, while raising the voltage to the max voltage level. This helps provide a full saturated charge, and repopulate the ions to the plates, but also limits the Ah sizes the chargers can recharge. Since your battery isn’t too far outside of that rating, it’s just going to take a little longer to charge. Please keep in mind the Repair mode is only for 12V Repair. However, it’ll still maintain the battery and the sulfation is still broken up to help repair it in the first steps of the charging cycle. The chargers will remove the charge once it is complete and go into a low energy maintenance mode where they monitor the battery. If the battery voltage drops below its resting voltage, the charger will provide a trickle charge to top the battery off. This is a better maintenance charge for the battery than a constant float voltage that brands like Tender will use.”


    I too have a NOCO 3500 charger and it is GREAT, although it does take its time to charge a 6V Optima battery.

    It has to go through all of its cycles before it lets go and indicates that the battery is charged.

    It will bring the Optima up to 6.2V / 6.3V and then blinks GREEN at you (final stage of charging) for an interminable duration.

    I recently KILLED my two-Optima 6V Red tops (? a switch was on or some other unknown reason for a draw?) and the NOCO 3500 would not recognize the DEAD Optima battery.

    I connected my two Optima 6Vs to a 6V Wet-Cell for 24-hours, or so, and that brought the Optima batteries up to a point where the NOCO 3500 would recognize the batteries.

    Then, it brought one up pretty well.

    I then charged them one at a time.

    I left one on a 6V Battery Tender and even that did a FAIR job, but not like the NOCO.

    EESCH, I hate it when something happens to kill the Optima battery .

    Also, I sent NOCO an email about whether the 3500 is good for charging AGM batteries, and they DID NOT REPLY!

    Optima sells very nice chargers, but NONE that have a setting to charge their 6V battery!

    Go figure that one out!

    From now on, when I am out on a tour or out for a long ride, I will have my two Optima batteries in the battery box, but with only one connected.

    BTW, if you attempt to use a 12V jump pack THROUGH THE BATTERY, you can FRY the gauges and anything else in line.

    That would be BAD!



    Great info. Thanks!

    1. Disconnect my jumpers and see the individual voltages

    2. George, I may have overkill. I replaced weak 6v lead acid battery. Thought having more amperage would do better. Perhaps I should remove one.

    3. James, the 1.1 Amp NOCO is holding the two batteries at a good charge.

    4. Peter, as you, I did have one 6v Optima that completely died and would not charge. I hooked a fresh battery to the dead battery and hooked my NOCO charger to the fresh battery and it brought the dead battery up to take a charge.

    These replies are giving me food for thought, thanks again!


    Hi Bill.

    There is no reason to ‘not’ use a pair of Optima’s in your ’16. But usually the big 6 cylinder engines start well with one battery.

    The main reason I believe is that the engine crankcase and transmission are aluminum, not iron. And the ground [+] cable is attached to the transmission as well as the steel chassis. Aluminum conducts electrical current much better than steel or iron.

    Ed Minnie said that the cars he’s in charge of have dual Optima’s, but are set up to use one at a time. This to me is really a great idea: just one light switch forgotten or leaving the transmission in reverse. or a brake pedal not returning to the ‘full up’ position . Any of these will result in a dead or very low battery overnight. With Ed’s idea: his cars would have ‘at standby’ another charged optima ready to have cables transferred and put to use. I really like that idea and method. A ‘wear a belt AND suspenders’ approach.

    I have managed to leave more than one of my cars in reverse , and kill the battery.

    One think Optimas DO NOT LIKE is to be fully discharged. Even using another 6v battery to get the charger to work on one of my ‘killed’ batteries, one just would not recover from my goof-up.

    Take care,

    Greg Long

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