Battery tender

**campernurse2012** · 12-08-2023, 08:14 PM

I was wondering what brand of battery tender is best for our TM? We would like to maintain the charge during the winter months and we were wondering what everyone else uses. TIA

Wavery · 12-08-2023, 08:37 PM

Quote:

Originally Posted by campernurse2012

I was wondering what brand of battery tender is best for our TM? We would like to maintain the charge during the winter months and we were wondering what everyone else uses. TIA

The most dependable and safest is a 20-25W solar panel with a small solar controller.

**Bill** · 12-09-2023, 07:55 AM

That is a good solution. It can produce about 2 amps in clear sun - and very little in cloudy weather and none at night or indoors, of course. Assuming there is AC power available, there are a number of plug-in battery maintainers (not trickle chargers) that work well. I have had good luck with a small Deltran Battery Tender Junior, for example. If you do a Search of the Electrical sub-forum using the term "maintainer", you will find recommendations for other units.

The important thing in all cases is to start with a fully charged battery, then disconnect it from the TM entirely, to avoid all the parasitic loads that any RV has, and then to connect the panel or maintainer.

Bill

**rickst29** · 12-09-2023, 12:34 PM

For the case of lead-acid batteries, the 'maintainer' should keep the battery at 100%, but must not ove4rcharge it. The voltage which corresponds to 100% changes according to temperature, and all the good ones have temp sensor lead.

If you do not fully disconnect the TM batteries from TM phantom loads, you will need more than 2A of current capability in float mode.
- - -

Lithium batteries should not be kept near 100% during long-term storage. Instead, 40-60% of fully charged "State of Charge" is optimal.

Most maintainers which claim to be 'lithium compatible' will try to overcharge the batteries, but that can be overcome by setting lower voltage BMS limits (i.e., the maximum voltage during which the the charge circuit remains 'enabled') for storage use.

They cost a bit more, but the original "battery tender" devices have fewer complaints among amazon buyers.

**Bill** · 12-09-2023, 01:52 PM

Rick makes a good point about lithiums. Being strictly old-school, I automatically think in terms of lead acid batteries. I would be a very poor custodian of lithium batteries - so listen to guys like Rick who know.

Bill

Shane826 · 12-09-2023, 02:46 PM

Bill- Is it you who has an adapter to charge your battery through the Bargman 7-pin connector? Got any pics or details how that came to be?

**Bill** · 12-09-2023, 05:48 PM

Yup, that was me. I did it on my 2006 2720SL TM - haven't done it on my newer one. I'm trying to find my notes. I will post when I do.

Bill

**Bill** · 12-10-2023, 09:44 AM

OK, found my notes, and discovered that some years ago, TM made a small change in the 12-volt charge line wiring. The new arrangement makes charging through the Bargman a bit different, but not really difficult.

The Early picture in the sketch below, redrawn from the 2002 Owner's Manual, shows the wiring in my 2002 and 2006 TMs. The 12-volt charge line from the Bargman goes directly to the battery. A second wire goes from the battery, through the main fuse, and out to the TM circuits. This means that if you pull the fuse to isolate the battery, the connection to the Bargman remains intact and you can charge the battery through it.

In the Later picture, redrawn from the 2008 Owner's Manual, the 12-volt charge line goes to a metal terminal strip in the converter, and then onward to the battery through the main fuse. If you pull the fuse, the battery is disconnected from everything, including the 12-volt charge line. Now simple charging through the Bargman doesn't work. The answer is to leave the fuse in place, and use a somewhat larger Battery Tender to carry the parasitic loads. A unit rated for 2 or 3 amps should be adequate.

My apologies to anyone who was misled by more recent posts on how to do charging through the Bargman. I was not aware that TM made this change.

Bill

**Bill** · 12-10-2023, 12:56 PM

To finish the story...

I use a Deltran Battery Tender Junior. It is a plastic block that plugs directly into a wall socket. The 12-volt output is a 10 foot two-conductor cord with a connector on the end. It comes with two shorter cords, each with a connector that mates with the long cord. One of these short cords has ring lugs on the far end, the other cord has jumper cable clamps.

From an RV parts place, I bought a 7-pin Bargman Car End connector, which mates with the 7-pin plug on the TM big black cord. I opened it up, identified the screw terminals that carry the battery charge line (12 VDC and ground), and screwed the Deltran ring lugs to those terminals. The other five terminals are unused. Reassembled the connector. Done.

It has been nice to pull the main battery fuse out of its holder, close the TM, back it into the garage, and leave it. At the front of the TM, I plug the Battery Tender into the wall, and plug the TM's big black cord into the new Bargman Car End. For a short time, the Battery Tender blinks red to show it is topping off the battery, then blinks green to show that the battery is almost there, then goes to solid green to show that it is in maintenance mode. It stays solid green for as long as it is connected. And the TM is always ready to go with a fully charged and properly-maintained battery.

A somewhat larger Battery Tender will give the same results, at a slightly increased cost.

Bill

Tireman9 · 12-19-2023, 02:29 PM

I believe that "Battery Tender" is a registered trade name from Deltran.

I have been using two BT Juniors on my RVs since 2008. One on the chassis and one on the coach battery. Currently both my batteries in my 2015 RV test good with a "Load Tester" despite both being 8 years old.