Battery question

[DanL]

Sorry, this is an R/C boat question, but thought the electronics folks here would have an answer....

I was planning to use two battery packs, a 6V and a 12V (each 4.3Ah), in a scale boat. The 12V pack to power the motors and 6V pack to power all the lights and animation. The 6V pack is 5 NiMH cells, the 12V is 10 cells (total of 15 cells).
The boat is getting too heavy for for both battery packs - they are heavy because the cells are 4/3AA size.

To save weight, I'm planning to go to two 6V packs (total of ten cells). Question - can I run the motors off the two packs in series for the 12V I need for the motors, and also split off separate 6V circuits from each of the packs individually for lights and animation?

Note - the two separate ESC's/motors draw less than 0.5A each at 12V, full speed, with props in water

Circuit planned shown in pic.

[Mr.RC-CAM]

It can work, but is not recommended. The shared pack will be discharged faster than the other. If you attempt to continue to run the boat after the shared pack is fully discharged, it may experience reversed currents. This is essentially a reverse charging condition. That is not healthy for it.

Ideally, you want all the series connected cells to see the same currents so that they are charged/discharged equally. However, if you are diligent, and stop the boat before a full cell discharge {on any one cell}, then your idea will work. Also, you will need to set it up so that each pack is charged on its own.

I suggest you use a separate pack for lights and animation and go worry-free.

[rick.benjamin]

"The boat is getting too heavy for for both battery packs - "
You might consider lithium packs. Lighter, smaller, with similar capacity

[lazy-b]

Guys: How about just use a 12 Volts and use a High Efficient 12 Volts to 6 Volts DC to DC converter......you might get it at Electronics Surplus Shop.....if your electronics is just a low power just use a Linear Regulator LM317, to drop 12Volts to 6 Volts.

[DanL]

Thanks all.
LazyB, how much current can an LM317 put out? Could I use more than one in parallel if I need the current capacity?
RC Cam - could I slip some diodes in the circuit to prevent reverse current to the batts?

[Mr.RC-CAM]

How about a $5 5V/1.5A DC-DC VReg? It is very efficient: http://www.rc-cam.com/dc-dc.htm

RC-CAM

[lazy-b]

DanL, Current Capacity for LM317 is 1.5 Amps, you may use LM350 for higher capacity its 3 AMPS, LM338 is 5 AMPS or LM396 is 10 APMS......Better use the DC-DC module posted by Mr.RC-CAM, if you can get one.....LM317 is so common, its available in all electronics store.

I guess its useless even if use a DIODE, this is common problem with all Multi cell Battery pack. thats why we have a cell Balancer....if your battery is not discharge evenly....the stronger battery will act as a CHARGER and the weak Battery will act as a LOAD, thats why it will reverse charge a weaker cell.

it look like most Scale Boat use a 6 Volts Sealed Lead Acid Battery.....I have a electric Speed Boat its ATLAS 540 made by acedemy...I use 12 cell Lithium-ion configured at 2S6P its equivalent to 7.2 Volts 8400MAH.....I can use it for more than an 30 Minutes....its really expensive when you buy it new, but able to get it at very cheap price at Surplus store, cost me only USD 1 per cell, its 1400MAH 3.6 Volts, the battery is design for old Cellphone.

[DanL]

RC Cam-
Ordered 2 of the MB-80 5V DC-DC converters at $5.50 each. Great reference article - taught me a lot. Thank you!
Some Q's, please: Is12V input within input rating (delivered setup is 9.6V in)?
Will 12V in yield a higher output V? Really would like to get full 6Vout as LEDs, lamps and accessories already wired with dropping resistors for brightness, speed, etc at 6V.
Is RFI going to be a prob in long power leads (can use 18 or 20g wire)to lighting, etc?
Is there a cheap way to filter the output and get cleaner DC?
Thanks much,
Dan

[Mr.RC-CAM]

Is12V input within input rating (delivered setup is 9.6V in)?
Not a problem.

Will 12V in yield a higher output V?No. This is a regulated DC-DC. Output is fixed.

Is RFI going to be a prob in long power leads (can use 18 or 20g wire)to lighting, etc?It is always wise to keep leads as short as you can. Twisted pair is nice too. Beyond that, range testing will determine if you have issues.

Is there a cheap way to filter the output and get cleaner DC?
The output ripple is very low. This DC-DC supply is not a budget design. Do not worry about this unless you find range testing problems when the supply is turned on.

RC-CAM

[DanL]

Thanks RC Cam and all - you are a great help and it's much appreciated.
DanL