Battery Tender question

[rotti]

Let me preface this by saying when comes to electricity I'm not the sharpest knife in the drawer.

I just installed a harness to my truck battery to run my Battery Tender this winter. The truck and camper are only a little over a year old and the batteries, one truck, two AGM's in camper, are in excellent condition and fully charged. The camper is on the truck full time.

I hooked everything up and expected the Battery Tender to show a flashing green light (80%) or a solid green (100%) right away as the batteries were fine to begin with. Now 16 hours later the light is still solid red indicating a full charge mode to a low battery even the battery(s) is fine.

First question, apparently the the tender is also charging the camper batteries as they show 12.8 volts at rest and over 13 volts when the BT is plugged in. The truck battery shows 12.7 volts at rest.

I guess my concern is if I am doing any damage as it seems the BT is over charging the system? I did check the BT on my Tahoe and Harley and it went to flashing green in less than an hour so it seems the charger is operating correctly.

Bottom line: Is the three battery set up confusing the BT?

 

[KILR0Y]

Personally, I'm Not A Fan Of Leaving Anything Plugged Into A BT. Just My .02 Worth...

 

[takesiteasy]

The issues I have experienced are two: the battery tender might time out before all the batteries are fully charged, and depending on the differences in size, one or more of the batteries might not ever get fully charged. I installed a switch to disconnect my camper battery from the truck system so I could charge it independently. Once it is fully charged, I can connect it back to the system and the charger runs in maintenance mode. I do run it in maintenance mode on just the camper battery for a couple of days every so often.

 

[rotti]

Thanks for the replies and it appears that takesiteasy is correct.

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.

 

[NorCalSteve]

I really have not researched how a battery separator works but, if the the battery tender is connected to the truck battery and the battery separator sees the truck battery is charging, it will connect the truck battery in parallel with the camper battery(s). My "guess" is that a relay engages and connects the two batteries together (although this could be a non mechanical relay). The relay would draw power from the battery just to keep the camper battery connected to the truck battery (to power the relay). This could consume a fair amount of the battery tenders power that really was targeted for the battery and not to power other things.

Takesiteasy set his up so the battery separator would not engauge when the battery tender was connected to the truck battery. I am guessing this is a reason why he did that. It seems to me that the relay could "possibly" draw .1 amp and keep the tender from ever switching to the lower voltage until the 8 hour period.

I should probably do more research before posting this but what the heck.

Steve

 

[takesiteasy]

Takesiteasy set his up so the battery separator would not engauge when the battery tender was connected to the truck battery. I am guessing this is a reason why he did that.

Exactly. I have the IBS battery separator and it would connect the batteries automatically when it sensed the charger current. I put the switch in to disconnect the camper battery so I could charge it independently. It's a pretty simple solution. I have the model of the Battery Tender that can charge at 8 amps, so it charges the camper battery pretty quickly and I can reconnect it to the truck to keep both batteries conditioned.