It depends a bit on what type of generator and regulator you have. With the simple and stupid cut out on the 3 brush splitdorf generator it is the battery size (amp hours) that is a vital part of the charging system. And it has to be an old type wet lead battery. The internal construction of the generator determine that because the field coil is using the battery in symbiosis as a buffer, up to a limit. A smaller battery (and even orig. battery) it at a greater risk of boiling out. 

Orig. battery is at (was it 19AH or 16?) However, that is probably of larger capacity of any of the modern batteries you find today on the market that will fit the battery box.

Slightly better chance of preserving the battery is with the 2 brush Bosch generator and regulator that senses and regulates the current that goes to the field coil. It is possible and I recommend to modify the Splitdorf to a 2 brush system with a sensible regulator.

Best chance to preserve a wet lead battery and even run with smaller capacity batteries, perhaps even sealed or gel batteries, is to use an electronic regulator that is a lot quicker and more sensible to the draw and what the battery can take in regard of charging current. There is a few on the market, both for the Splitdorf and Bosch. 

A fair warning is entitled. Regardless if you use an old generator or decide to run some of the alternators that is small enough to fit on the Indian, a standard electronic regulator is not suitable for a modern fancy metal, ion hydrid or hybrid battery variants that is easy accessible on the market these days. Those kind of batteries needs a specially adapted for the type of battery, fast acting and exact electronic regulator. The batteries could be fire or exploding hazard when overcharged, as also gel batteries are in some cases. BUT I don't know if such specialized regulators are available on the market for those needs. 

So, a wet lead battery is most common to use. Today's batteries is more delicate constructed with thinner plates and stuff inside so they are prone to vibrate to bits and short out internally, in the battery box. With a hard tail as the 101 is, the jolts and vibrations are tremendously violent on the pot hole riddled roads of our "modern days"! Use a tilers knee pad under the battery and loosely on its sides to isolate from vibrations. I even made a coil sprung pad under the battery in order to have a longer shock absorption range!! Still the crappy batteries of today doesn't last long...

I have Gene Harper's electronic regulator with a small cheap sealed (AGM) battery, which has lasted forever.  These batteries are much more rugged than a flooded battery because the plates are sandwiched tightly between glass mats.  They can't tolerate overcharge for long because the tiny amount of electrolyte will soon be lost.  I would not use an AGM battery without an electronic regulator.

Hello,

I have a 1929 101 with a DU5, and have read the posts regarding the issues of over charging of the DU7.

I assume that the DU5`s have the same issues of over charging as the DU7?

From Carl-Erik`s comments, would I be correct in assuming that it is the higher the AH of the battery, that acts as a buffer to prevent the battery from overcharging/frying?

Are the Gene Harper Regulators still available?

Will the Gene Harper Regulators fit in the DU5?

The attached is a Lead acid battery that will fit in the battery box, however it`s only a 6AH, but Capacity notes 10 AH. Would this battery be suitable if modified to 2 brush setup?

Any feedback, opinions or alternatives would be greatly appreciated.

Thank you in advance.

A very large selection of (AGM) batteries are offered by Power Sonic Corp in San Diego CA see website POWER-SONIC.COM, up to 210 Ah!.

But only a few AGM's are 6 volt I see now.

I have use their 6N 11A-1B conventional lead acid battery with a Bosch DVAR with original regulator.  Its old and I will be purchasing one of their AGM batteries (My DVAR has a been reconditioned by Antonio Esteves and has a new regulator). Only the PS-6100 FI seems suitable and is widely available in U.S. ( next day delivery from Amazon stock where I am.

Years ago, on another 101, I was using their PS-6100 F1 AGM battery with originally regulated DU7 (2 brush). This now has a alternator disguised in a DU-7 case.