The Charger Choice

I think, by the time I’m done, I should put together a collection of all the wonderful puns that the whole electric vehicle thing uh, “sparks”.  I’ve already nixed about 4 charger-related ones in the last 2 minutes…  but I digress.

The charger is a pretty basic component in the system, a fairly simple tradeoff to get your head around, and one of the less exciting decisions you’ll be making.  In fact, one article I read said that many builders leave this to the last, and get a build put together without any good way to charge it.  (Guilty as, uh, charged.  D’OH!)

If you want the final word on battery charging, look no further than Battery University’s section on charging, battery types and principles. Go here, about halfway down the page.

The basic realities of battery charging go like this.

1. Match the voltage and battery type to the charger.  The voltage part is pretty obvious, but the battery type is crucial  Every battery chemistry wants to be charged with a different profile- metering out the charge you’re feeding to the state of charge of the battery, over the charging time period.  Some chargers are programmable, and in come cases you can use one charger for different batteries- but you really need to confirm the charger will match up with the batteries you’re intending to use.  You could kill your battery investment in short order if it doesn’t.

2. Speed of charge is everything. Small, light, inexpensive chargers are low current- that takes longer to charge your pack.  If you want fast charging, you’re going to pay more money, and the thing is going to be bigger and heavier.  Fast charging also strains the batteries more, too.

Simple enough…  fast charging = more money, weight, size and stress.

Variations: If you’re running a pack with a lot of little lithium cells and a BMS, you can run a big charger for the pack voltage- say, 72V.  If you want to experiment, don’t want a BMS for your charging, or are running a pack with, say, six 12V batteries, you can try using individual chargers on each cell.  In that case, you’d use one of these, for example- this is a 3.2V charger for lithium cells, and you’d wire this to each paralleled cluster of cells in your pack.  Or, you can use the lead battery version, and you’d have six of them- also a common scenario.

Since each charger is set up to top each cell group up to the standard target voltage, you get a really even state of charge across the pack.

In the case of lead packs, this is a pretty common solution. The chargers are pretty small and cheap, they take some time to charge, but since lead packs rarely run a BMS it’s a good way to even out the charges.  In the case of lower voltage cells like lithium (usually 3.2V) where you generally are running a BMS to protect the batteries on discharge as well as charge, it’s not as common.  Here’s Ed “Juiced” Fargo’s description of his charger array on ElMoto.net.

Another interesting variation is the solar charging option.  SunForce has a huge array of what is now pretty affordable solar battery charging products. Solar panels are a very basic cost/capacity tradeoff as well, so if you’re willing to spend some money and get some big panels you can get enough current to charge the bike in pretty short order.  If you want to keep the size and cost down, that’s fine, and you’re going to get a longer charge cycle.  Just one thing- you’re not likely to get enough charge out of anything that you can carry onboard the bike.  We’ve tossed around all sorts of ideas about incorporating panels into bike covers or foldout deals, but the bottom line is they’re just not big enough to be practical.

These products can come with their own convertors and charge controllers, so it’s a project of it’s own- but running the bike 100% off the grid?  Priceless.  Once again Ed “Juiced” Fargo and the Solar Shed, from ElMoto.net.

The On-Board Mounting decision: This is an interesting question…  do you want to mount the charger on the bike?  Here’s how this plays out.  If you have a small charger, it will fit on the bike easily.  It will, however, take a long time to charge.  If you have a bigger charger, it will take up more room, weigh more, but charge faster.  If you have a huge charger, it won’t fit on the bike.  If you want to have a charger onboard so you can make your daily commute, for example, and charge up at work, then it’s not as tough a decision- you can settle for a nice small charger.  If, however, you want a charger for backup, one that, if you get stranded and need to recharge fast, you can plug in for a half-hour and make it home, then, you’re in a kind of a pickle.  You’ve got to figure if the added size and weight of the charger offsets the range of batteries it displaces.

For my rig, I started out running no on-board charger so the bike would be as light as possible.  I figured with a short range anyway I am charging at home mostly all the time- I go out, hammer on the bike for a few miles, then charge it up in the garage.  I figured I could carry the chargers (I run one for the 72V pack, and one for the 12V batteries- a disadvantage of using a separate battery pack for the 12V system) in a courier bag if I needed them.

Within a month of riding, I had them mounted on the bike.  It’s just more convenient.

Some mounting notes, though…  chargers are not at all waterproof.  Your controller may be, your motor may be, and your wiring will be if you do it right, but the charger can’t get wet.  It also needs cooling, so anyplace you mount it to keep it protected and dry may make it overheat.  Those little brick-type chargers, though, are a lot more weather-resistant.  Also, keep in mind, the chargers really weren’t built for a high-vibration environment.  If you’re doing lots of miles on rough terrain, then you may want to consider keeping your charger off the bike.

Here’s an interesting story that illustrates the charging conundrum.  I wanted to build a bike that could go 200 miles in one day.  The frontal assault on that is to run a ton of batteries.  More batteries is more range, but takes a really long time to charge with a small charger.  So, if I have a pack that that can hold out for 200 miles, I have very little room on the bike, and my little charger will take 12 hours to recharge it.

A pretty creative option is to run a smaller pack, and instead of the batteries run a charger that can recharge my pack in a short time.  For the sake of discussion, let’s say I run a pack that goes 50 miles, and a charger that recharges it in 1 hour.  I ride 50 miles, stop for breakfast, recharge.  I ride, stop for an hour, ride, stop for an hour, like that.  In an 8-hour day, I get 4 hours of riding 50 miles- there’s my 200 miles, and I get 4 one-hour rest stops.

There’s one thing you have to consider, as well- that’s how you’re going to connect the charger.  You want to disconnect the charger when you’re not charging the pack, and when you’re running a BMS you have to connect through that.  The simplest solution is what they use on forklifts- you have a big Anderson-type connector coming from the pack.  When you’re charging, you plug that into the charger.  When you’re running, you unplug it from the charger and plug it into your controller circuit.  It’s a little, well, manual, but it’s safe and simple.

Wanna see what the big boys use?  Here’s the charger from the Brammo race team’s first year:

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