Rechargeable Battery Box Alternative?

[Legoroni]

Does anyone know of any 7.4V battery that can be used in a Power Pickup System? All the batteries I found are either 3.7V or cannot be charged while running. I ask because the train I am trying to build cannot fit 8878. Thanks

Also, how would the performance of the train motor change if it was only running off 3.7V?

Edited June 3, 2014 by Legoroni

[peterab]

Also, how would the performance of the train motor change if it was only running off 3.7V?

Since the PF system is in principle designed to run at 9V, 3.7V would result in the motor running much slower, in fact I'm not sure 3.7V is enough to get it to move at all.

[Duq]

Can you put two battery packs in daisy-chain? Or is that too big again?

I'm sure the motor will run on 3.7V but it'll be slow. If you want to use the PF receiver you may have a problem. I don't think that'll work at all on 3.7V.

[spzero]

Hobby King is probably the best bet buy a stnadard Li-po from a radio controlled vehicle (obviously a small one) they are 7.4 volts and in use they generally would not be charged while working will work with the right voltage going in etc, LEGO use Li-po's at a lower volt as the next step up would be 11.1 volt and too high but they give a higher output compared with regular batteries so performance wise is not too dissimilar, you might be better getting a Li-fe battery or even nimh's as they would be more reliable and could easily be found from hobbyking at reasonable prices small ones from micro cars or helicopters etc are your best bet.

[legoman666]

Why can't your batteries be used while charging? The current flowing in to and the current flowing out of them would cancel (to varying degrees). Put a couple diodes in there to keep polarity correct, and you'll have no problems. For example, you'd have 300mA from the batteries feeding the motors, and (for example) 500mA charging the batteries at the same time. The net effect on the batteries would be a 200mA charging current. If you stopped the train, the 300mA drain on the batteries would stop and the net charge being applied would be 500mA.

My main loco picks up 24V from the rails, passes it through a bridge rectifier, and feeds a DC-DC converter that chops it down to 10V for a PF IR Receiver. Additionally, there is a 7.2V battery pack (6 NIMH rechargeables) protected by a diode that also feed the same DC-DC converter when the train is on plastic rails. If I had a DC powered battery charger, I could use the track voltage to top up the batteries when the train was powered rails (alas, I do not).

[jtlan]

Can you put two battery packs in daisy-chain? Or is that too big again?

Since the battery produces 3.7V, I assume they are Lithium-Polymer cells. My understanding is that some specialized knowledge is required to safely connect them in series (cells that are not properly "matched" may overheat and explode). You can buy premade two-cell packs that produce 7.4V (one of these is inside the Lego battery).

Why can't your batteries be used while charging? The current flowing in to and the current flowing out of them would cancel (to varying degrees).

Lithium-Polymer cells require specialized charging circuitry (plugging them directly into a power source will result in flames and much sad for you). The Lego rechargeable pack contains a charger inside, which is why it can be "charged" by plugging it into the wall (incidentally, this also explains its cost).

@Legoroni, you may be able to find a NiMH battery pack that will fit inside your model. You'll still have to buy some specialized charging circuitry. In addition, NiMH packs are much heavier, although that may actually be beneficial for a locomotive.

Can you give us a diagram of your intended electrical setup?

[nils]

All my trains run perfect on 4 AA rechargeable NiMH batterys (with a capacity of 2450 mAh). That is 4.8 V. In curves the top-speed is too high for staemloco's, so speed enough.

I use very cheap battery holders. They fit in a space 8 studs long, 4 studs wide and 3 bricks and a plate high. The IR receiver can manage 2 PF trainmotors.

[legoman666]

Lithium-Polymer cells require specialized charging circuitry (plugging them directly into a power source will result in flames and much sad for you). The Lego rechargeable pack contains a charger inside, which is why it can be "charged" by plugging it into the wall (incidentally, this also explains its cost).

I thought this would be obvious. Just hooking up an unmetered power source to any battery type is a terrible idea. That's why you need a charger to manage the charge current.

[Legoroni]

@Legoroni, you may be able to find a NiMH battery pack that will fit inside your model. You'll still have to buy some specialized charging circuitry. In addition, NiMH packs are much heavier, although that may actually be beneficial for a locomotive.

Can you give us a diagram of your intended electrical setup?

I plan to make power pickups out of metal wheels, which would charge the battery, powering an IR receiver and train motor, along with lights.

I think I will search for an alternative battery type that will fit and try to do what legoman666 is doing.

[JopieK]

The LiPo from LEGO is just too great to ignore. It is so solid and well protected! we use it at school and believe me students have been harsh on them. you could use a protected power pack like these though: http://www.dx.com/s/18650+3.7

you will need separate chargers for them, but they are well protected agains shorting etc., only they are quite large (larger than AA.

[locoworks]

why not use PP3's, you can wire them in parallel to get higher currents if one battery doesn't give enough power.