Battery Tender on Dual Battery Setup

[chris belyea]

I've read post on using a single battery tender on a dual battery setup. I've read that you need to connect the batter tender to the positive on one battery and to the negative on the second. My question is, if you have a battery tender with cigarette adapter can you use that or do you really need to connect directly to both batteries to insure they both get charged?

Regards

Chris

[RSTarga]

I am no electrician, but you only have one alternator.

[edmayo]

The path for the car's charging system is from alternator to starter terminal, then thru cable to driver side battery (left side), then thru cable to passenger side battery, (right side). So by hooking the charger to only the left battery you are charging both batteries the same way as the car's system. This is the way I hook up my maintainer, there may be an electrical reason this doesn't work well, but I don't claim to understand it.

[lopena]

Ed is correct (as usual)...just connect the battery tender to one battery and the other one will also get charged.

Alan
N.J.

[JackMan]

Thanks Ed, great knowledge to have.

[NorthernThrux]

Actually Ed is only partially right. The correct answer is it depends on the type of charger you use. Smart chargers connected to only one battery adjust their voltage and cycle based only on the primary battery load. For trickle charging of two fully charged batteries, this isn’t such a big deal, but if you are doing this to charge the batteries, especially if unevenly depleted, then tender to positive on one battery and tender to negative on the other is correct.

The difference is that the alternator isn’t smart like CTEK or other chargers and doesn’t load sense the way the smart chargers do.

See for example,
http://www.batterytender.com/connect...el-one-charger

Ravi

[G69]

When the two 12V batteries in the Porsche long hood are connected in parallel as from the factory they are acting as one battery (bank).

No multi-battery smart charger would be able to differentiate between the two batteries when they are wired in parallel.

Isolating the batteries by disconnecting any one terminal on either battery and then the multi-battery smart charger could then delegate voltage to both batteries so each is maintained but never over-charged. These multi-battery smart chargers offer an elegant solution to shops where multiple customer's batteries in storage are in different conditions.
Each battery gets its own custom charge regimen. And each battery has its own lead(s) from the charger...

What is easiest for our long hoods is keeping the batteries in parallel as from factory and always hooking up a simple smart charger like the Battery Tender to one battery or the other when not using the car for a month-

Marine systems that have two batteries often have a master switch that can choose battery 1, battery 2, or both in parallel.
This way batteries are never left in parallel unless being maintained by shoreside chargers or alternator(s). This allows running one battery (battery 1) at anchor with lights, stereo and fridge and having another (battery 2) as a spare should we run battery 1 down below where it can crank engine. When discharged below what is necessary to start an engine the user can parallel the two batteries on the 'ALL' position on the switch to get the engine started and then charge the both batteries while still in parallel mode with the engines alternator.

Note that discharging a lead-acid battery like the ones supplied in long hoods below half capacity or 12V causes permanent loss of capacity. Remember 12.6 is fully charged. 12V is actually discharged...

When Porsche designed the dual battery setup- they were concerned in getting the batteries weight balanced and as far forward as possible to offset rear weight. As long as the cars were daily drivers the batteries got along fine.

Once these relics were parked for months this setup became problematic-

Batteries in parallel fight each other to the bottom. Batteries in SERIES do not- Two six volt batteries in SERIES would eliminate our problem...

When ever a battery (one cell or multiple cells in SERIES) are connected in parallel one battery is always in a higher state of charge than the other. Never in the real world are the states of charge truly identical-

And there is our problem. Without a maintenance charger plugged-in, the weaker of the two batteries will pull current from the other.
This will continue until both are fully discharged.

Boats handle this by using a main battery switch to disconnect the batteries in parallel from each other when not in use or not being charged by alternator or shore-power charger. Many big boats do not use big automotive sized batteries in parallel- They achieve the needed capacity with HUGE six volt batteries in SERIES- That way the parasitic loss from a parallel connection is eliminated.

The only simple answer for long hoods is to either disconnect one terminal (POS or NEG) from either battery before long-term storage, or leave them in parallel and connect a battery tender to insure the common voltage is above 12.6V so one cannot bleed the other.

As long as the original factory wiring is used, the battery terminals are clean and tight, this works well.

The charger's connection can work with any hookup as long as the batteries are in parallel, the positive lead of the charger is on the positive side of either battery and the negative is on either battery negative or the CHASSIS.
I often hook my charger's NEG lead right to the chassis-ground wing-nut on the drivers side battery.

The splitting of hookup as mentioned above may help if a terminal isn't clean and tight, or if the original wiring has been 'modified'- otherwise parallel is parallel. The electrons will find their way home.

Remember an open circuit in the batteries will ruin the 911s alternator's diodes immediately- A good reason to keep it SIMPLE.

The best solution is to DRIVE the cars weekly so the alternator can catch-up on this parasitic loss and bring both batteries up to almost full charge.

That is how the cars were designed to operate.



It is not a design failure. It is from not using the cars as designed.

[NorthernThrux]

When the two batteries are close in charge, then a tender keeps them both charged. Current flows from the stronger battery to the weaker one (as noted above), but the tender keeps the "donor" topped up because the internal resistance of the two batteries is approximately equal and internal resistance depends on voltage that the battery is producing.

When one battery is heavily discharged (low voltage, say 10V), its internal resistance is easily triple that of the fully charged one. This means that two thirds more of the current flows through the charged battery than the discharged one (because the voltage drops are the same by definition, V=IR and all that. Higher resistance, lower current). Smart chargers are measuring the max current and that is determined primarily by the resistance of the charged battery, not the discharged battery when in parallel. So once the smart charger detects that a predetermined current is flowing (dominated by the low resistance of the charged battery), it switches to a trickle mode (so as not to boil the electrolyte) and you never get enough current to charge the weaker battery. This does not happen with an alternator, which regulates voltage, not current.

If your passenger side battery is discharged and you try to charge it using a smart charger on the driver's side, it will take you forever to top the passenger side up, if ever. If you use an old fashioned charger that just puts out 14V, it will charge back up, just as an alternator would do so.

Li Ion batteries don't have the same problem because their internal resistance is fairly flat, regardless of charge. Lead Acid and AGM batteries have significant increases in resistance as their voltage drops.

[RSTarga]

I am having a hard time understanding how two batteries wired in parallel could have such different states of charge, while running off the same alternator. My experience has always been if one is dead so is the other.
Even if one was defective, it would pull the other one down to its level very quickly.

[NorthernThrux]

I am having a hard time understanding how two batteries wired in parallel could have such different states of charge, while running off the same alternator. My experience has always been if one is dead so is the other.
Even if one was defective, it would pull the other one down to its level very quickly.

They wouldn't. Unless you disconnected one for a while or replaced one. I went through this exact issue this summer. Only one battery was in the car, even though 2 had been purchased. Tender was wired into the driver's side and when I installed the passenger one, I couldn't charge it initially. The tender would just go into trickle charge mode. Switched the charger to the passenger side and it charged right up and since then, the tender keeps them both charged.