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Old 05-18-2019, 01:25 PM   #1
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Two Battery Chargers

I install a inverter charger on our Mainship 390. I turned off the charger on the inverter because the boat already had a Sentry 12v 60amp 3 output charger that charges my house bank 2 8D AGM, Start battery 8D flooded and Generator start battery. My question is would it hurt to have both the inverter charger and the Sentry charger on at the same time to help the batteries recover quicker at anchor when using the generator?

I also have a Blue Sea SI-ACR automatic charge relay with start isolation install for charging when underway.


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Old 05-18-2019, 09:26 PM   #2
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Won't hurt anything. They will work together. Confirm the chargin profiles match your battery type and are at least similar.


My inverter/charger is a 1990ish Heart Freedom 25 130amp. It drops to float at 21 amps which is way too high. The batteries would never get fully charged.


I also have Promariner charger that drops to float at about 3amps. They work fine together.
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Old 05-19-2019, 12:42 PM   #3
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Thanks Wood

The inverter has 2 different AGM settings but the Sentry charger which was install in the boat when I bought it does not have a adjustment that I can find. Look like the Sentry charger floats at 13.5 v so I’ll set the inverter for the same and see what happens.

Thanks again
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Old 05-19-2019, 12:48 PM   #4
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I have used two identical chargers to get higher output with no problems.

I believe that the larger charger will cause the smaller charger to reduce or end charging. The small charger sees the voltage output from the larger charger, assumes battery is charged and reduces or stop charging.
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Old 05-19-2019, 01:34 PM   #5
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It is what I do.

When I added an inverter-charger I kept my older charger.

On the hook I will run my 14 Kw Onan which has plenty of power for the two chargers working in parallel on the house batteries.
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Old 05-19-2019, 02:00 PM   #6
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It is what I do.

When I added an inverter-charger I kept my older charger.

On the hook I will run my 14 Kw Onan which has plenty of power for the two chargers working in parallel on the house batteries.
Are the chargers the same output?

Have you measured output of the chargers while running parallel?

What I found was both chargers may run parallel initially but the larger one will overwhelm the smaller charger as the batteries come up.

It may be brand specific.
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Old 05-19-2019, 04:48 PM   #7
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Are the chargers the same output?

Have you measured output of the chargers while running parallel?

What I found was both chargers may run parallel initially but the larger one will overwhelm the smaller charger as the batteries come up.

It may be brand specific.
Until the bank is fairly charged-up it accepts all the current the two chargers can throw at it.
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Old 05-19-2019, 06:43 PM   #8
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Until the bank is fairly charged-up it accepts all the current the two chargers can throw at it.

I would think each charger would send its max configured current until they sensed that the absorb voltage had been reached.
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Old 05-19-2019, 09:10 PM   #9
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I do the same, running both my Balmar alternator (120A) and my Honda powered shore charger (55A) to maximize my charge when needed. They play together very well.
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Old 05-19-2019, 09:23 PM   #10
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Well that debased knowledge I picked up somewhere.

This is why this forum is so useful, sort the fact from fake advice.
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Old 05-19-2019, 10:24 PM   #11
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Similarly, Solar charge controllers don't ask the charger for permission to add charging amps, they just do it. The charger doesn't complain. The batteries benefit.
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Old 05-20-2019, 05:31 AM   #12
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Most chargers monitor the voltage to decide on output to add.

If the batts are way down and the voltage is low the chargers will work together to boost the charge.

When the 85% charge (or so) is reached the charger with the highest voltage will do the charging the other will simply watch.

Using one charger for one bank and the other for a different bank can sometimes speed the last 15% of charge a bit.

Todays practice of a massive house bank works great for discharging , but a rotary switch to split the house bank in two may be a faster way to charge that last bit to get to the required (for long life) 100% charged.
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Old 05-20-2019, 06:55 AM   #13
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Add me to the dual charger club. My Freedom30 inverter side went TU, and I wanted a reason to go to a Full Sine inverter anyway, so rather than deep six the Freedom30, I relocated it next to the new Magnum, and wired a smaller set of conductors to the battery buss. I set up the profile for maximum absorb voltage. Both chargers work together happily in absorb, when the bank is down around 65% SoC they both crank up to near max, so I'll sometimes see 230A going into the bank. It's AGM, so it's happy to take all I can throw at it.

The result is faster charging, and with the solar to top off the bank during the day, I'm getting to 100% SoC almost daily. The bank performs much better, higher capacity, and I'm pretty confident this bank will outlive the predecessor Lifelines that I effectively murdered through chronic undercharging. 90% SoC and done will do that.
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