Electronic – Rechargeable battery type recommendations for a 1-2 amp bicycle LED circuit

batteriesvoltage-regulator

We are building a blinkey-blink circuit for a bicycle and not sure what battery to use.

  • The circuit will need to run on about 5V, since is is powered by ATmega chip which must run at 20mhz.
  • When all the LEDs are on, the circuit will consume about 10Amps (yes, there are lots of LEDs). We anticipate that about 1/8 (12.5%) of the LEDs will be on at one time, so we are estimating 1-2amps current draw.
  • Ideally the circuit could run for a couple of hours on a charge.
  • The battery should be relatively small. Lugging around a car battery on a bicycle is not an option.

The LEDs are SMD RGB LEDs, being driven from TLC5940. There are 120 LEDs, and 24 TLC5940s to drive them. There are a few other components, but other no major current sinks.

According to some posts here (which I am having trouble finding at the moment) LiPO batteries provide the best power density among the rechargeables. I worry that with LiPO batteries we'll have to use voltage regulators to get our 4.5-5.0V, since LiPO cells are 3.7V each. Does that mean losing power to heat? I suppose switching voltage regulator is an option, but I know very little about them.

What would be the recommended battery types for this purpose?

Best Answer

(This answer summarizes Anindo Ghosh's suggestions on the topic made in EE chat plus a few of my own observations. Please note that I am not considering this answer definitive: just want to add something else to the mix of suggestions)

This answer assumes (without justification) a LiPO battery-based design. Some background information:

  • LiPO battery cells are 3.7V
  • Connecting battery cells in parallel is undesirable because if one battery has lower internal resistance than the others, the first battery will discharge first and then the other batteries will begin charging the battery with the lowest internal resistance.

The recommendation suggestion is to split the circuit into the following parts:

  • 5V circuit: for the ATmega and all the LED drivers. This part will be driven by a switching boost regulator to step up the voltage from 3.7V.
  • Separated the LEDs into banks such that each bank is driven by set of serially-wired LiPO cells: this avoids having the LiPO cells wired in parallel (more on the reason for serial wiring below).

Since the TLC5940 is a sink driver, the LEDs can be wired at whatever voltage. Because these are RGB LEDs the \$V_{forward}\$ of at least some of the green and blue channels can be as much as 3.4, leaving only 0.3 V of headroom assuming given LiPO's nominal voltage. Adding the fact that LiPO voltage varies substantially over the discharge cycle (Wikipedia: "The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged)"), this is not sufficient. Furthermore, according to Anindo Ghosh and a TI thread, TLC5940 requires a good amount of headroom over the \$V_{forward}\$ voltage to properly regulate the LEDs: \$Vcc_{LED}\$ must be greater than \$Vforward_{LED}\$ by about 1.2 Volts at 120mA sinking current - see Figure 5 of the datasheet. This leads to the conclusion that each LED bank must be driven by two LiPO batteries which will yield a minimum 5.4 volts to drive the LEDs; after the \$V_{forward}\$ drop of 3.4 V there is still 2V of headroom left.

A possible alternative to consider is witing R, G, and B channels differently: since the R channel has lower \$Vforward_{LED}\$, it may be wired from a single LiPO cell, while G and B channels will still be powered by two serailly-connected cells.

Finally there was a suggestion to consider LiFePo4 batteries, which have tighter output voltage over the discharge cycle, offer longer cycle life and higher peak current.

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