Thursday, 5 April 2012

Battery Charging

Recently Andy (boatbuilder) informed us he had placed an order for our Beta 43 engine and it should arrive at the end of April.  This has caused me to revisit the battery charging requirements.  We will have three battery banks on Waiouru.

  • Starter battery (1 x12v)
  • Bow Thruster battery bank (2 x 12v)
  • Domestic battery bank (4 x 6v @450Ah)

The Beta 43 engine comes with two alternators (1 x 65A & 1 x 175A).  The starter and bow thruster batteries will only get infrequent use so it shouldn’t take much time to recharge them.  This means the 65A alternator will not be under any significant load for most of the time.

The domestic battery bank has a capacity of 900Ah and the 175A alternator will not have sufficient output to charge the battery bank.  Especially since we plan to cruise for an average of 3-3½ hours daily.  We need a charging system that can charge the domestic bank within that timeframe.

Initially the plan was to combine the output of both alternators using something like the SmartBank Advanced system.  Andy isn’t a fan of split charging systems and I’m also starting to think relying on the 65A alternator may not be a good idea.  It’s designed for starter motors requiring short term heavy loads, rather than continuous maximum output loads.  It’s therefore likely it could prematurely fail.

Even if they were combined (65+175) we would only get 240A (less efficiency loses).  What we really need is 33 to 50% of the battery bank capacity.  That is, 300-450A. 

The Beta can be fitted with a 240v Travel Pack which produces 3500 watts.  We could run this back through an inverter/charger and combine it with the 175A alternator.  However is seems to be very inefficient to produce 240V AC and then convert it back to 12V DC.  Besides, there appear to be a number of boaters who have been forced to have their Travel Power unit rebuilt after a couple of years use.

My latest thought is to substitute the Travel Power alternator for a second 175A alternator.  The two 175A alternators could then be combined to give 350A (less efficient loses).  This is within our generating requirements.  Another option might be to replace the 175A alternator with a larger unit.  I’m not sure if this is possible and have emailed a query to Beta Marine to see if they have previously completed any similar modifications.


Bruce in Sanity said...

Hi This makes no sense at all to me; how much power are you going to use in a day? That's what's important, not the total size of the domestic bank.

We have 4 x 110 Ah domestic batteries, and the standard 175 A alternator. I use maybe 25% of the bank's capacity on a hard day, and that's recharged in around 2.5 hours engine running.

If you're going to need much more than that, you should think about a cocooned standalone genny in the engine bay; much more efficient than third alternator!



Anonymous said...

Don't forget that you will only be charging the battery bank to replace what you have used overnight. I know it varies but I have a 550Ah domestic bank and typically it will go down about 10 - 15% from late afternoon until mid-morning the following day. The boat has both a fridge and freezer and these are the big comsumers of power. Incidentally, my domestic alternator is 95A and recharging the battery usually takes 2 - 3 hours. (I realise that often the battery is not completely recharged since the last few % can take a long time.) Hope this helps in your consideration of capacities.

Tom and Jan said...

Hi Bruce,

You have hit the nail on the head. The other key factor is the power consumption! If we both use the same amount of domestic electricity each day then, with the size of our battery bank, I will only need to run my engine every second day.

Using your charging figures I'd need to run the engine for 5 hours every second day. But I only want to run the engine for a maximum of 3.5 hours.

Therefore; do I need a larger alternator output. I think the answer is probably YES!

I've now read some technical information from two sources. One quotes a charging factor of 5 and the other a factor of 4.

Depending upon which source you believe a 900AH battery bank requires either a 180A or 222A alternator.



Dave Winter said...

I don't know if this might help or confuse but it's worth a read....

Tom and Jan said...

Hi Dave,
Thanks, we are aware of the information on Peter's blog and actually lived on Kelly-Louise for two months.

I completed a detailed power audit for Waiouru during the planning process several years ago and have conservatively estimated daily usage at 120VA. The battery bank should last 3-4 days without needing recharge.
It's the matching of the alternator output to the size of the battery bank in the desired time frame I'm pondering about.

bargemast said...

Hi Tom and Jan,

your alternators will normally never be able to charge 175Amp for more then a few minutes, after which the Amps will reduce.

Good luck with sorting something good out.

Happy Easter,


Tony Q-J said...

Hi guys,

Firstly, your 175A alternator is plenty big enough - the bank will only accept full charge amps for quite a short while as the previous poster pointed out. Thereafter the current will slowly decrease.

Secondly, there is absolutely no way that you will fully replace the charge used with a 3.5 hour charge every 4 days; you will kill your batteries within a year. Take a read of this:


Tom and Jan said...

Hi Tony,

Interesting website. I note they state for a "kind charging regime use 25-30% of the battery capacity and the writer uses 35-50%.

With our battery bank of 900Ah the alternator output therefore needs to be 225A-270A for a "kind" charge and the writer would uses 315-450A accepting a slightly shorter battery life.

A single 175A alternator is only 19% of our battery capacity.

Tony Q-J said...

"A single 175A alternator is only 19% of our battery capacity."

Yes, but 39% of usable capacity.

Let's say you've dropped your batteries all the way to 50%, so you have 450Ah to replenish. Your 175A alternator can supply 450Ah in just over 2.5 hours but way before that the batteries will be demanding less than 175A. They will continue to demand less than that for several hours (a lot more than the 3.5 you quoted - more like 8 or 9) until they finally approach 100% charged.

If you were to use a Smart Bank to parallel the alternators then you don't really need to worry about the 'extra work' that the 65A alternator will be doing - it would only need to be in parallel for around an hour or less before the batteries dropped their demand to below 175A. 65A alternators are used to working for hours on end to supply all the electrical demands of today's modern cars.

Another consideration is the size of cable you'll need for a larger alternator. You'll be wanting something like 90mm2 with the existing 175A alternator if you wish to actually get most of that power into the batteries, and a larger alternator would require correspondingly larger cables.