Battery Tender question

X2
install a cutoff switch on the camper batt & wire the tender directly to batt. The alarms/sensors will keep the tender going forever.

It isn't the fact that they are paralleled.

It is the fact that you have a load on the batteries at the same time you are trying to charge them. The charger may have a smaller output than the parasitic load. Disconnect the batteries from the load and it will charge any number of batteries successfully.

All seems normal to me. Let it run.

For what it is worth: The BatteryMinder Plus can charge up to four batteries at once. We used one at a 3000 tree commercial peach orchard we owned in CO to keep four farm equipment batteries charged in the winter.

Seems like Battery Tender is telling you that you should buy another or even two more!

Thanks for all your replies and to clarify I do have the Battery Tender Plus which has an output of 1.25 amps.

I found the following on the BT site:
(I assume my truck battery and two camper batteries are considered to be connected "in parallel"?)

from batterytender.com
14. Can the Battery Tender Plus battery charger be used to charge more than 1 battery simultaneously if the batteries are connected in parallel?

Yes, the Battery Tender Plus battery charger can be used to charge more than 1 battery simultaneously when those batteries are connected in parallel. Theoretically, there is no reason that you cannot recharge your batteries in parallel, or that you can’t use a larger battery. HOWEVER, you must recognize that the amount of time required to recharge may be much longer than you would normally expect. Effectively, by charging more than 1 battery in parallel, the charger behaves as if one larger battery is connected to its output terminals.

The Battery Tender Plus only puts out 1.25 Amperes. That means it will take over 24 hours to recharge a 32 Ah battery to 80%, assuming that is fully discharged. It will take another 12 to 24 hours on top of that to recharge the last 20%. If you put an even larger battery in parallel with it, then the total times may double or triple. That is not a reason for concern.

The real concern is that the Battery Tender Plus will only switch over from 14.4 VDC absorption voltage when the current draw from the battery drops below 0.1 amps or after an 8-hour period in absorption mode. Under normal circumstances, with battery capacities up to 32 Ah, this is a good thing and the Battery Tender Plus will switch over to the long-term storage voltage of 13.2 VDC with no problem. In fact, it usually turns out that the amount of time spent at the constant voltage of 14.4 VDC, typically a few hours, is good for the battery, especially the newer AGM style batteries. There is a maximum time limit of 8 hours at 14.4 VDC. As long as the charger switches over to 13.2 VDC before the 8-hour timeout, then the battery will be 100% recharged. However, the larger the battery that you try to recharge, the higher likelihood that the charger current will never drop below 0.1 amps with 14.4 VDC applied, no matter how long the charger is connected. That means that the charger output will remain at 14.4 VDC for the maximum time period of 8 hours. This is also not a problem for the battery in terms of "dry-out", but again with the larger batteries if the charge current has not been reduced to a maximum of a few tenths of amps, then there is a good possibility that the battery will not be 100% recharged before the switchover to 13.2 VDC. This will result in even more time required before 100% recharge is achieved.

Our recommendation is that you not charge batteries in parallel, again assuming that the batteries are 100% discharged. If the batteries are only partially discharged, then it is probably OK to charge them in parallel.

W/ my tender,it appears to charge at the hi voltage for 4 hrs minimum, no matter what. So if you interrupt the charge at any time, it may go red or blinking for 4 hrs.

Most of those little battery maintainers don't have enough juice to keep more than one battery fully charged, especially so if there are parasitic loads present.

I have a two amp maintaner, and I left it for a few months on two 95Ah group 24s that were connected in parallel, but were removed from the RV. It boiled them almost dry and ruined both.

Bought a new set identical, and charged them not connected in parallel, with the same maintainer and had no problem. I then decided to read the manual for the maintainer, and it has a max Ah battery that it will charge & maintain, and two 95Ah together (190Ah) was too much.

So.....I'd suggest having a look at the manual of your maintainer. 3 full sized batteries connected together may be too much, fully charged to start or not.

Which Battery Tender? 0.75 amp? 1.5 amp? 5 amp? 20?

It would appear all is fine. Let the charger work. It may not have the power to get the batteries up to about 14.6 volts to drop into float. So it will harmlessly charge away for about 72 hours and then drop to float.

Just make sure voltage rises on all batteries to verify voltage is getting across the isolator (if any).

You don't specify so I will guess you have the Deltran Battery Tender Plus. If so it is only 1.5 amp maximum charger. If you have any parasitic loads on the battery which I'd wager you do, it will never be able to fully charge.

Disconnect the batteries from load and the BT will fully charge them. Understand what I am saying? You are using a very small charger and trying to charge at the same time you have radio(s) and sensors trying to discharge.