On Sat, Jul 20, 2002 at 09:07:47AM -0700, Jim wrote: > Thanks Byron > > What I'm using it for is a 3 wheel robot that I'm building. Right now the > battery is powering a pic16f874 and just a few circuits including a sn754410 > hbridge and 2 small swiss motors that free running at 12v draw less than > 100ma each ( I haven't checked them under load) but I will be powering a ccd > camera on the bot later > and some sensors and oh yeah 2 small pan & tilt motors for the camera. > I have to admit I'm a fly by the seat of my pants engineer but the battery > that I'm using seems large enough > to power all that and more for a few days but 12 hour min would be fine. I > thought I understood ah ratings this battery says (5ah/20h) that's a little > confusing. The 5Ah is the capacity of the battery. The 20h is the rate of discharge that that capacity was measured. Batteries are funny creatures. The amount of power that you can get from a battery depends on the rate that you take it. The slower you take power from them, the more power they will deliver. Just a quick example: the capacity rating implies that you could get 5A@12V for 1 hour. However that's 20 times the 20h rating. So I gurantee that if you drew power from the battery at 5A that it would conk out well before the 1 hour had elapsed. You may get 10 minutes, maybe a bit longer. So the time rating indicates the rate you can draw on the battery for 20 hours to get 5Ah worth of current out of it. So in this case it's 250ma. I'll go ahead and answer your other post here too: This looks like a deep cycle application since you're planning to run your robot for awhile on a single charge. Most likely float charging won't be sufficient to recharge to capacity. Hmmm, You already have a 874 right? So you have the A/D module available. Good. Do you have enough I/O left for a one A/D, 1 digital input, and two outputs? If so then you can build the battery charger control into the bot itself. Here's how: 1) Build your charger so that it has the LM317, the voltage adjust resistors (an R2 for 14.4 and an R2 for 13.8) along with current limiting power resistor limiting to 4A or so. If possible get a LM317K, which is in a TO3 case and can handle up to 5A. 2) Take you battery power and run it through a 2 to 1 resistor divider (with x ohms on the bottom and 2x on the top) , which will take your battery voltage and divide it by three. Throw in a 5.1V zener for good measure to ensure that the input won't be damaged by some stray high voltage. feed this into one of your A/D inputs. 3) So now your charger has a 4 line interface: * V+ that goes to your battery * A 14.4V control input that's connected to one of the PIC's output pins. * A 13.8V control input that's connected to another of the PIC's output pins. * GND Finally you need an indicator to the PIC that the charger is plugged in. I'm not sure of the least invasive way to do this. SOme ideas are an optoisolator or a relay that is driven by the charger. Or possibly another resistor divider going to a digital input. In any case the bot will need a steering diode between the charger V+ and the battery so that when the charger is disconnected the bot can sense the charger side without the battery voltage disappearing. That's all the hardware. The software is simple. Use the A/D to measure the battery voltage. Charge at 14.4V until the battery's voltage rises to 14.4V then switch to 13.8V until the charger is disconnected. It's only a 2 stage charger (bulk and float) but it's certainly better than simply float charging the battery. To add the absorption phase would require conditioning and sensing the voltage across the current limiting resistor to determing the amount of current that the battery is drawing from the charger. But two stages is better than one since the bulk charging phase will get the battery to close to 90% charge. Jim, I'm sure glad that you brought this up. I think that with a judicious choice of a resistor divider that it may be possible to do the voltage sensing with a 16F628's analog comparitors and its internal programmable voltage reference. My new game plan is to incorporate the 16F628 into the charger. I still have one outstanding question that I can't seem to get a clear answer on. When discussing bulk charging every document that I've seen discusses constant current and that the voltage of the battery rises. However it's unclear to me what are the allowable voltages during the phase. In other words is the 14.4V the max voltage allowed during the bulk phase? Some documents have quoted 15V. A constant current charger is easy enough to build using an LM317. However it's unclear what the input voltage for a bulk charger. Here's the specific quote from this page: http://www.salt-systems.com/marine-batteries.htm "Bulk Charge is the first stage of battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take." So does one simply use an unregulated high current supply? Or is the max for the charger the max current at 14.4V? BAJ > > Jim > > -----Original Message----- > From: Byron A Jeff > To: PICLIST@MITVMA.MIT.EDU > Date: Saturday, July 20, 2002 8:38 PM > Subject: Re: [EE]:speaking of gellcell's ? > > > >On Sat, Jul 20, 2002 at 06:57:05AM -0700, Jim wrote: > >> Hi > > > >> Pardon my ignorance > > > >No need. You have very cogent questions. > > > >> I'm working on a home project using a 12v gellcell. > > > >One piece of information that's missing here is how are you using the > >battery. What type of charging you need depends on how it's being cycled. > > > >> It has charging info printed on it but I'm kinda new to charging these. > >> It say's cycle use 14.5 ` 14.9v (25deg c) initial current less than 2 > amps > >> stand by use 13.6 ` 13.8v (25deg c) > > > >Typical... > > > >> > >> I have been looking at charging info on the web > >> I would like to use an lm317 for the charging and have already > breadboarded > >> a cicuit that will switch between the 2 voltages > >> How important is the temp compensation? > > > >It's important if there's a wide variation of temperature from 25C. > > > >> Can I charge it at around 1 amp without any ill effect? > > > >Not at 14.6V. This voltage will guarantee an overcharge if left > indefinitely. > > > >>if the charger is working properly should I be able to measure aprox 14.6v > >>when I first connect it to the battery or does it have to climb to this > >> voltage as it charges? > > > >The latter. The battery will drag the voltage down to its level as it takes > >on charge, slowly rising to the target voltage. > > > >Battery charging is serious and complicated business. Essentially charging > >under any conditions other than ideal will eventually result in battery > >failure: > > > >* No charge will result in a dead battery. > >* Undercharging will cause sulfation resulting in lowered capacities and > > premature failure. > >* Overcharging will result in gassing causing premature battery failure. > > > >I would advise not charging your battery from 14.5V source unless you have > >a an automatic cutoff mechanism (or two) in place. I finally decided that > >float charging was the safest dumb charging course of action. > > > >A good reliable gell cell charger is high on my list of things to build. I > >really want a 3 stage charger with autodetion of stages (bulk, absorption, > and > >float) and decent indicators. What I really haven't figured out is how to > get > >really high current for fast charging. My 33Ahr 12V gel cell will accept up > >to 10A current. But finding a 10A transformer is a tough proposition. > > > >Hope this gives you some help, > > > >BAJ > > > >-- > >http://www.piclist.com hint: To leave the PICList > >mailto:piclist-unsubscribe-request@mitvma.mit.edu > > > > > > > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads