Direct Battery Connection or via a Controller

[Usman]

I was wondering what would be the real benefits of using a charge controller with charging methodologies i.e. PWM, Trickle battery charge, MPPT etc.. over direct connection of +ve -ve wires from the turbine to the batteries, apart from controlling over/under charging.

Assuming the overcharging and over discharging is taken care of, is it that using one of these controllers would make a slight or drastic differnce over direct connection to the batteries?

A few controller manufacturers use terminologies like "faster charging" or "most sophisticated way" or "smoothly" or "in most efficient way" etc.. really mean something? or direct connection is as efficient?

Thanks.

[tecker]

You want to match the geneset to the load to control stall , stop runaway and as you say check over charging . The control can be a simple design or way complexed to take into effect the the a really large battery bank or small unit . There's  quiet a few threads here that are very  informative . Your individual VA and usage have to be spread out in detail to get any good feedback .I don't see any problems starting without a controller if the stator wire is large enough . Watch over your battery and take care to be able to shut the turbine down to keep it from running away . Match and balance .Stator over heating is an issue with close attention to phase balance  and parasitic currents.  

[Hank]

I'm no expert by any means and as you bring up an interesting question here's my two cents.

A charge controler is just that, it controls charging and is not a load controler although maybe there are some units that do both.

So as your question is about charge controlling or charging in general I'll talk about that.

First of all efficiency is a tough one to get a hand on as I'm not sure what that usually means.

In a perfect world a direct connection to the batteries would be the most efficient as any time you add another component into a system it likes to get paid so what goes out is less then what goes in (no free lunches ever).

Reality on the other hand is quite different.

You just can't throw all the charging current into a battery and expect it to charge up. Oh it will but if the current is to high you'll ruin the battery in short order ( this may happen in a direct connected system).

In terms of charging I lean towards (and have the best results) with a PWM three step charging process.

In the first step (Bulk) the charger will put in the max amps the battery will take to bring it up to a bulk voltage setting.

In the second stage (Absorption) current will be varied (usually decreasing as the battery carges) to maintain the bulk voltage.

In the final stage (Float) current is reduced and varies to keep the batteries at the float setting.

Excess power is diverted elsewhere and if not used it is wasted.

Do a Google search on batteries and you'll find a wealth of info on charging.

Of course a lot depends on your battery capacity and charging system, be it solar, wind, hydro or el-cheapo battery charger.

If your charging system is less then C5 (5% of amp-hour battery capacity) then you probably don't need a charge controller. Above that you run the risk of ruining your batteries without a charge controler (human intervention is a form of charge controller).

Boy that was a mouth full, hope it helps.

[Flux]

Hank has covered it pretty well, If you have a decent sized battery for the charging current then up to about 80% charge you can manage to take it all. If you have a battery that is small in relation to charging current you may need to limit current at all times.

If you connect direct, you just can't watch it all the time so unless your battery is very large you will damage it.

All charge controllers sense voltage and if you keep a battery at constant voltage it will take what charge it can manage automatically. Any excess current over what it can use is just diverted to heat.

Mppt makes better use of available current during the bulk charge mode from solar panels, it can not be used with wind unless specially designed. It provides a better match between the source and battery. During the constant voltage regulating phase you don't see much advantage as you are throwing away excess energy that you have acquired more efficiently.

There are several ways to control at fixed volts. The simplest method is to turn on a load at the desired voltage and turn it off when the voltage falls a few mV.

Pwm does exactly the same thing but at a high frequency ( khz) and the battery sees it as more or less a constant voltage.

You can maintain a true constant voltage with a linear regulator or a modified pwm with inductor and freewheel diode. It seems form most studies that batteries are happier with the fluctuating pwm or on/off charging than perfectly constant voltage ( the exception being sealed batteries on float).

Unless you have a large battery and are prepared to spend a lot of time watching it as it nears full charge then some form of control is almost a necessity. Modern batteries will not stand the abuse that the old Plante cells had to stand before the days of controllers.

Flux

[Usman]

Thanks to you all, Tecker, Hank, and Flux, for your feedbacks.

I wrote in my comments that I have the overcharging and discharging taken care of by means of a voltage monitor, that turns on a dump load at a 56V (for my 48V system) and disconnects the loads when the voltage drops to 43V. It is done by means of Solid State relays.

All I am worried about is what happens in between. The 48V 2 KW PM turbine (that was a Chinese type of brushless alternator) has a descent 20KW lead-acid battery-bank. My concern was that if a direct connection would charge in the most efficient way or at least in a descent way!

Can you recommend an off-the shelf PWM or three stage charger for a wind turbine? A lot of stuff available for solar but nothing seems to show-up for a wind turbine? Instead of speculating if a certain controller, that was originally meant for solar, would match the turbine, I would rather connect it directly to the batteries. But then, a second thought raises question on efficiency of such a connection and if this would cause amps wastage and inappropriate and inefficient use of the turbine's full capacity. Any suggestions!!!

Thanks, Usman

[Flux]

I am not sure what is bothering you. Ignoring MPPT for solar, all controllers let the mill charge the battery directly.

If you have a dump load and low volt cut off then you are covered.

If you buy a commercial controller it will do exactly as you are proposing except that if you use it for diversion control then it will not deal with your low volt cut off.

The mill will charge directly to the battery until the volts reach about 14v nominal then the controller will divert excess current so as to maintain that set voltage. Some do it just as you are proposing with a diverter resistor switching between 2 close voltages. Others do the same thing at a high frequency as pwm ( they can make it cheaper that way and batteries don't mind).

Three stage charge is a bit of a refinement in that it does the pwm at say 14.3v and when the batteries have been at this voltage for a certain time the controller decides they are fully charged and then reduces the voltage to a lower level say 13.8v.

If set at 13.8 permanently the final stage of charging will be very slow and the batteries will never truly be fully charged. If you set at the bulk charge and leave it there for weeks then you cause excessive corrosion of the positive plates.

The 3 stage control is ideal if you leave the site unattended all week and use the batteries at weekends, it will get them back up quickly and float them for the rest of the week.

You seem concerned with every bit of charge so it is likely that you are going to use load often and even if you have the 3 stage charge you may not see it drop to float very often. If your thing is capable of satisfying the requirement of holding the bulk charge level and will also deal with the low volt cut off then it will be ok.

Determining state of charge is far from easy and at best most of the controllers make an inspired guess as to when the batteries are fully charged and when to drop to float.

Unless you are in a situation where you will be holding the bulk charge for weeks at a time the 3 stage will not be a great benefit. If you set a bulk charge at about 14.2 and equalise after a big discharge or perhaps every 6 weeks otherwise, you should be fine.

Some batteries are fussier than others and you should try to find the actual voltages from the battery manufacturer.

Flux