To deep-cycle or not to deep-cycle, that is my question :P

Deep-cycles ARE traditional lead-acid batteries. They're just made with thicker plates, which means they aren't capable of the same amperage as, say, a car battery, but the electrodes aren't damaged as badly in cyclic use.

The discharge curves are different for different brands of batteries, but it's easy to find a spec sheet for specific batteries; websites of battery makers usually have them posted up somewhere on the site.

How many Ah you get out of them depends more on construction and maker for a given chemistry, methinks. It doesn't end up being much of a difference, anyway. The most I've seen (for example) half U1 size vary is from 17Ah to 20Ah.

The reason you want to use proper deep-cycle batteries is so they last more than a few dozen cycles. The kind with spongy plates don't last very long when used cyclically.

"Spongy plate" is the kind used in car batteries. They have more surface area than a solid plate, so can deliver higher amperage. The tradeoff is that they don't take to deep cycling at all, because the plates get deformed and sulfated quicker. In use, this isn't much of a problem; they only have to run for a few seconds before the car starts and they get recharged. But you'd be lucky to get a dozen or so deep-cycles out of a car battery.

AGM is the good stuff. I think they last the longest of all types of lead (except for maybe silicone. I dunno much about those). They're what's used on pretty much every scooter/bike that uses lead.

AGM is the good stuff. I think they last the longest of all types of lead (except for maybe silicone. I dunno much about those). They're what's used on pretty much every scooter/bike that uses lead.

I believe this to be a relatively true statement but I would point out that Silicon is not as forgiving as AGM and their life is much shorter if you do not treat them exactly right. For this reason alone I would use AGM over Silicon any day. I have also read reports where members have replaced their Silicon batteries with AGM batteries and had better performance from their scooter.

Grandpa Chas S.

AGM is the good stuff. I think they last the longest of all types of lead (except for maybe silicone. I dunno much about those). They're what's used on pretty much every scooter/bike that uses lead.

I agree that AGM is the lead-acid battery type to use for scooters and ebikes but other lead-acid technologies do appear to have better cycle life. Gel batteries which make good wheel chair batteries have a longer cycle life, but these have a higher resistance (or impedance) and won't deliver as much capacity in a high-rate application.

In regards to the topic:

I got a twin kollmorgen bike that I'm building and is nearly completed. The motors are 24v with a 21v battery safety cutoff. I know that deep cycle batteries have more amp hours than normal batteries but If the voltage drops off the same it wouldn't do me any good. What is the voltage curve to amp drain like in deep-cycle bats compared to traditional 12v lead-acid batteries? What would you recommend? Thank you. -Ryan

To answer a couple of your questions.

I know that deep cycle batteries have more amp hours than normal batteries but If the voltage drops off the same it wouldn't do me any good.

This is actually incorrect. For the same voltage cutoff point, batteries designed more for deep cycles tend to deliver less ah because of the difference in construction that LinkofHyRule pointed out.

What would you recommend? Thank you.

I recommend B&B batteries of either EVP (high rate, high cycle use), or EB (specifically designed for electric vehicles). You can check the spec sheets for these, and B&B provides power time tables for both. At a cutoff point of 10.5v, the EVP12-12 (12v, 12ah) deliver 94.4% of the energy in whrs as they do when it is 9.6v. This is when discharged in 30 minutes. If you check the discharge curve, after they hit 9.6v at about 12 amps, the voltage drops real quick, and you won't get much extra capacity beyond that.

BTW, why not just go NiMH or NiCad? These batteries would cost about 2.5-3x as much but the weight would be a lot less for a bicycle. I would go with either the sub-C NiMH cells (can get RC packs with them), or SAFT D NiCad cells which are running kind of cheap at $6/cell for the D 5ah cells.

If voltage drop off is your concern, none of the lead acid batteries is the one you want. The nicads or nmih will be better. Lifepo4 is really good for providing the same voltage till just before it's all gone, but if you need a lot of power in a gulp, for a big fast motor, the duct tape lifepo4's arent able to do that. Lead will deliver a big jolt, but at the cost of making the battery seem half the size it actually is. Puekerts effect, which I couldn't explain. The spongy battery plates are found in the starting type batteries and look like lace hankies or snowflakes.

AGM. Check my above post for the manufacturer and type.

What size batteries are you going to use? I ask because I suspect trying to charge at 25 amps may be a bit too much especially with any type of sealed battery (AGM of Gel.) Using a rate beyond 0.3-0.4C is a bit risky when charging and may cause your batteries not to last as long.

B&B is da kine. A charger that outputs 25 amps is suitable only for really big batteries like golf carts use. That fast a charge on smaller batteries will definitely cook them to death, and could even be dangerous. Most scooter chargers put out only 2 amps or less which is too slow sometimes, at 6 to 10 hours. The commute you plan should be doable on 2, 18 to 24 amp hour size batteries if you can charge at work. That could double if you have a big hill to climb. That could double if you ride fast. The same battery that would go 20 miles at 12 mph on flat roads, might only go 6 miles on hilly roads at 25 mph. A battery pack in the 12 amp hour size may be a bit small since there would only be two. 18 ah or 24 ah may be the way to go, or connect 4 batteries to make a bigger capacity pack. In case you don't know this, amp hours is a measure of how much energy a battery can store. Amps is a measure of how fast energy is being used. Most ebikes get range in the ballpark of what you intend to do using 12 amp hour batteries in 36 or 48 volt configurations. That is three or four batteries in the 12 amp hour size. For me, the lithium was the way to go since i needed a lot more range for my commute, but your ride is well within the performance envelope of good lead batteries.

Re reading your original question, I see we may be answering your question but still leaving you unaware of some important items. Since ev's demand a lot of power in a short time, it could seem to you that a cheaper starting battery would be fine. The issue is not so much how much power can a battery deliver as much as how many times it can deliver the ammount you typicaly need. The spongy plate, starting battery will only deliver 80% of its stored energy a few times before it's ruined. A normal car start only uses a very small ammount of the energy in the battery. Ev's on the other hand need the type of battery that can deliver 80% of the stored energy at least 200 to 300 times. We try to buy a battery that will let us get where we need with about 70% or less of the capacity since that may get us into the 300 to 500 cycles range if we also charge carefully with a charger designed for battery life maximizing.
For electric cars, the battery of choice is often the same as golf carts use, mostly for economy when buying so many batteries. Trolling motor batteries, the most commonly avaliable deep cycle batteries are usually too big (heavy) for bikes and small scooters and also are actually designed for trolling motors that use a lot less power at a time than a bicycle or small scooter motor. Some of the wheelchair batteries also are designed to deliver the power slower than bikes require. The B&B batteries are ones with the best track record for getting a lot of discharge cycles. The types recomended in previous posts are designed for a good compromise between lots of cycles and delivering the power at the rate your ev will need. Getting the wrong type of battery can leave you with needing new ones several times a year, or reaching the cut off voltage too soon since you pulled the power out faster than the battery can tolerate. That case leaves you paying for and packing the weight of a big battery while only getting the power of a small battery. There is a website called battery university that may help you if you want to delve into the details of different battery types. It helped me a lot but remembering new knowledge is allways still a problem. You don't need to worry too much about the cut off voltage of your bike, since you really need to get big enough batteries to have the 20% reserve you speak of. Unfortunately finding out what your range will really be requires you to buy some batteries in the first place. Cut any manufacturers claims of range in half. Lots of other brands cost about half that of B&Bs but I haven't heard any raves for them yet. The cheaper ones are what usually comes in the scooters and bike hub kits from the factory. They work fine, especially for casual, recreational use. Serious commuters are using lithium, nickel, or the best of the lead batteries.