Recharging batteries while the robot is powered on.

Sticky

Could someone give me a fairly detailed description of how to charge a battery while a robot is still powered on? Is it as simple as attaching a charger to the batteries or should there be a filter or voltage/amperage protector of some kind?

I'm lazy and haven't really checked if there is a recent post for this, I hope there isn't.

Sandgroper

Well I'm no expert, but one way is to use a suitable switching circuit, (e.g.·relay, power transistor, etc).· When the robot connects to the recharge station and starts to power up, the switching circuit disconnects your main block and connects the recharging block.· When full, the switching circuit·flips back, disconnecting the recharge block and reconnecting your main circuit.·

This assumes that the robot doesn't need to keep operating while it's recharging.· If you only need the robot to retain data, you could get around it by shifting the data into EEPROM.·

If the robot needs to be ready to run at all times, you'd probably need a separate power supply.· The type of power supply would depend on the requirements.· If the current draw is low enough, you might get away with just·capacitor /resistor combination to·keep the chip ticking away while recharging is in progress.· Otherwise you·should consider·dual power supplies and trigger the·robot to switch·from the depleted source to the fresh source while recharging is in progress.

This raises the question of what's the best·strategy for recharging.· You need to consider how long the robot is expected to run for between charges,·how long it's able to recharge for, and how much human intervention is needed.· It might be more practical to have·a fully charged power pack on standby and just swap it out when required.· That eliminates the downtime associated with recharging.·

If the robot is doing frequent short runs,·it might be better to use a·smaller (lighter) power pack with a lower milliamp-hour rating and recharge it more often.· If the robot is doing long runs or heavy work, you'd need a larger (heavier)·power pack·with a higher milliamp-hour rating, which means longer recharge times as well.· The other thing to consider is the "C" rating of the batteries, which determines how much power·the batteries can actually deliver in one hit.

All of this determines·what type of batteries and charger you'll need.··Different batteries require different chargers.· Some are fast charge, some are slow charge, some produce more heat, some less.· Generally speaking the chargers are specific to the type of battery - either NiCad,·NiMh,·LiPo, lead acid, etc.· I prefer NiMh, but each have their pros and cons.

Manetheren

What are the "quick" pros and cons of each?

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Tia'Shar Manetheren

Sandgroper

LiPo's:··Comparatively new technology.··Relatively quick charge times.··Some concern over possible overheating / fires if incorrectly charged or charged in confined spaces.· Rapidly become the favoured battery type for high power, high demand applications, e.g. model electric racing cars.· Relatively high cost at this stage.

NiMh:· Very common, more mature technology, widely available, moderate cost, good performance.· No problems with "memory effect".·

NiCad: Older technology, known problems with "memory effect" if not completely discharged before recharging, which reduces battery life.· Some environmental / safety concerns over cadmium.

Lead acid:· Low cost, widely available, proven technology.· Good for heavy loads (e.g. starter motors, electric wheel chairs), long recharge times.· More difficult to maintain.· Risk of explosion if incorrectly charged.· Heavy.· Environmental / safety risks because of lead content and battery acid.· Arguably the battery of choice for heavier robots.

Timmoore

NiMh - they self-discharge, leave them for a few days and they loose a lot of their charge.

Lead acid - look at SLA (sealed lead acid).

Make sure your charger supports the battery type, many/most chargers dont support all types.

Mike Green

In addition to all this, in order to charge a battery, you have to provide more voltage than the battery normally supplies. Most chargers look at the amount of current supplied to the battery and the voltage across the battery. If you're still using the battery, the current drawn by the robot may confuse the charger (or throw off the charging process if you're not using a "smart" charger).

It's possible to design a charging circuit that would disconnect the robot from the battery when a charger is plugged in and connect the robot to a separate part of the charger for powering the robot while charging.

Scope

I disagree that lithium polymer batteries are always expensive. Here's a 25C 2200mah 11.1V for less than $20:

www.hobbypartz.com/liba25c2211t.html

P!-Ro

same price for a larger lead-acid battery for a robot that i'm working on right now. 270 amp output, used on snowmobiles and such. to put it simple, I disagree with you (don't take it hard)

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PG

Zoot

On my larger 'bots (self-docking, self-charging) I build the charger with an adjustable regulator (LM317/LM350 etc) on a big heatsink set up for constant current. I set the constant current values so that I have enough charge current to equal a slow-charge on the battery (say 1.2 amp for a medium sized lead acid) + whatever current the 'bot consumes when it shuts off most of it's peripherals, motors, lights, while charging. It's "on" but turns as much of it's features "off" as it can. Given that, say the bot consumes 300ma "feeding". 1.5 amps out of the charger seems to charge the battery fine.

An ADC monitors battery voltage to determine when charging is required/complete so the 'bot can back away from the dock (otherwise, with a constant current charger, the battery would overcharge eventually, though I keep current just above a trickle/slow charge range); a small zener is used on the 'bot side to an I/O pin to detect when there is voltage across the front of the charging "tusks".

Both 'bot and charger/nest are fused and protected by 6amp diodes so there can't be inadvertent short circuits across the charging hoops or the charging tusks. The diodes also help drop the voltage in just a bit to get it into a nice maximum limit for charging lead acid batteries at a constant current (I use a 2A 16v wall-wart supply scavenged from a old inkjet printer).

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When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php

Sticky

Thanks everybody for the input, gives me some ideas to use.

CounterRotatingProps

@Scope:
thanks for posting that site - I'd not seen it before ... some good, and reasonably priced stuff there! - H

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